diff -r 000000000000 -r 6474c204b198 layout/generic/nsFlexContainerFrame.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/generic/nsFlexContainerFrame.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,3358 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=2 et sw=2 tw=80: */
+
+/* This Source Code is subject to the terms of the Mozilla Public License
+ * version 2.0 (the "License"). You can obtain a copy of the License at
+ * http://mozilla.org/MPL/2.0/. */
+
+/* rendering object for CSS "display: flex" */
+
+#include "nsFlexContainerFrame.h"
+#include "nsContentUtils.h"
+#include "nsCSSAnonBoxes.h"
+#include "nsDisplayList.h"
+#include "nsIFrameInlines.h"
+#include "nsLayoutUtils.h"
+#include "nsPlaceholderFrame.h"
+#include "nsPresContext.h"
+#include "nsStyleContext.h"
+#include "prlog.h"
+#include <algorithm>
+#include "mozilla/LinkedList.h"
+
+using namespace mozilla;
+using namespace mozilla::css;
+using namespace mozilla::layout;
+
+// Convenience typedefs for helper classes that we forward-declare in .h file
+// (so that nsFlexContainerFrame methods can use them as parameters):
+typedef nsFlexContainerFrame::FlexItem FlexItem;
+typedef nsFlexContainerFrame::FlexLine FlexLine;
+typedef nsFlexContainerFrame::FlexboxAxisTracker FlexboxAxisTracker;
+typedef nsFlexContainerFrame::StrutInfo StrutInfo;
+
+#ifdef PR_LOGGING
+static PRLogModuleInfo*
+GetFlexContainerLog()
+{
+  static PRLogModuleInfo *sLog;
+  if (!sLog)
+    sLog = PR_NewLogModule("nsFlexContainerFrame");
+  return sLog;
+}
+#endif /* PR_LOGGING */
+
+// XXXdholbert Some of this helper-stuff should be separated out into a general
+// "LogicalAxisUtils.h" helper.  Should that be a class, or a namespace (under
+// what super-namespace?), or what?
+
+// Helper enums
+// ============
+
+// Represents a physical orientation for an axis.
+// The directional suffix indicates the direction in which the axis *grows*.
+// So e.g. eAxis_LR means a horizontal left-to-right axis, whereas eAxis_BT
+// means a vertical bottom-to-top axis.
+// NOTE: The order here is important -- these values are used as indices into
+// the static array 'kAxisOrientationToSidesMap', defined below.
+enum AxisOrientationType {
+  eAxis_LR,
+  eAxis_RL,
+  eAxis_TB,
+  eAxis_BT,
+  eNumAxisOrientationTypes // For sizing arrays that use these values as indices
+};
+
+// Represents one or the other extreme of an axis (e.g. for the main axis, the
+// main-start vs. main-end edge.
+// NOTE: The order here is important -- these values are used as indices into
+// the sub-arrays in 'kAxisOrientationToSidesMap', defined below.
+enum AxisEdgeType {
+  eAxisEdge_Start,
+  eAxisEdge_End,
+  eNumAxisEdges // For sizing arrays that use these values as indices
+};
+
+// This array maps each axis orientation to a pair of corresponding
+// [start, end] physical mozilla::css::Side values.
+static const Side
+kAxisOrientationToSidesMap[eNumAxisOrientationTypes][eNumAxisEdges] = {
+  { eSideLeft,   eSideRight  },  // eAxis_LR
+  { eSideRight,  eSideLeft   },  // eAxis_RL
+  { eSideTop,    eSideBottom },  // eAxis_TB
+  { eSideBottom, eSideTop }      // eAxis_BT
+};
+
+// Helper structs / classes / methods
+// ==================================
+
+// Indicates whether advancing along the given axis is equivalent to
+// increasing our X or Y position (as opposed to decreasing it).
+static inline bool
+AxisGrowsInPositiveDirection(AxisOrientationType aAxis)
+{
+  return eAxis_LR == aAxis || eAxis_TB == aAxis;
+}
+
+// Indicates whether the given axis is horizontal.
+static inline bool
+IsAxisHorizontal(AxisOrientationType aAxis)
+{
+  return eAxis_LR == aAxis || eAxis_RL == aAxis;
+}
+
+// Given an AxisOrientationType, returns the "reverse" AxisOrientationType
+// (in the same dimension, but the opposite direction)
+static inline AxisOrientationType
+GetReverseAxis(AxisOrientationType aAxis)
+{
+  AxisOrientationType reversedAxis;
+
+  if (aAxis % 2 == 0) {
+    // even enum value. Add 1 to reverse.
+    reversedAxis = AxisOrientationType(aAxis + 1);
+  } else {
+    // odd enum value. Subtract 1 to reverse.
+    reversedAxis = AxisOrientationType(aAxis - 1);
+  }
+
+  // Check that we're still in the enum's valid range
+  MOZ_ASSERT(reversedAxis >= eAxis_LR &&
+             reversedAxis <= eAxis_BT);
+
+  return reversedAxis;
+}
+
+// Returns aFrame's computed value for 'height' or 'width' -- whichever is in
+// the same dimension as aAxis.
+static inline const nsStyleCoord&
+GetSizePropertyForAxis(const nsIFrame* aFrame, AxisOrientationType aAxis)
+{
+  const nsStylePosition* stylePos = aFrame->StylePosition();
+
+  return IsAxisHorizontal(aAxis) ?
+    stylePos->mWidth :
+    stylePos->mHeight;
+}
+
+/**
+ * Converts a logical position in a given axis into a position in the
+ * corresponding physical (x or y) axis. If the logical axis already maps
+ * directly onto one of our physical axes (i.e. LTR or TTB), then the logical
+ * and physical positions are equal; otherwise, we subtract the logical
+ * position from the container-size in that axis, to flip the polarity.
+ * (so e.g. a logical position of 2px in a RTL 20px-wide container
+ * would correspond to a physical position of 18px.)
+ */
+static nscoord
+PhysicalPosFromLogicalPos(nscoord aLogicalPosn,
+                          nscoord aLogicalContainerSize,
+                          AxisOrientationType aAxis) {
+  if (AxisGrowsInPositiveDirection(aAxis)) {
+    return aLogicalPosn;
+  }
+  return aLogicalContainerSize - aLogicalPosn;
+}
+
+static nscoord
+MarginComponentForSide(const nsMargin& aMargin, Side aSide)
+{
+  switch (aSide) {
+    case eSideLeft:
+      return aMargin.left;
+    case eSideRight:
+      return aMargin.right;
+    case eSideTop:
+      return aMargin.top;
+    case eSideBottom:
+      return aMargin.bottom;
+  }
+
+  NS_NOTREACHED("unexpected Side enum");
+  return aMargin.left; // have to return something
+                       // (but something's busted if we got here)
+}
+
+static nscoord&
+MarginComponentForSide(nsMargin& aMargin, Side aSide)
+{
+  switch (aSide) {
+    case eSideLeft:
+      return aMargin.left;
+    case eSideRight:
+      return aMargin.right;
+    case eSideTop:
+      return aMargin.top;
+    case eSideBottom:
+      return aMargin.bottom;
+  }
+
+  NS_NOTREACHED("unexpected Side enum");
+  return aMargin.left; // have to return something
+                       // (but something's busted if we got here)
+}
+
+// Helper-macro to let us pick one of two expressions to evaluate
+// (a width expression vs. a height expression), to get a main-axis or
+// cross-axis component.
+// For code that has e.g. a nsSize object, FlexboxAxisTracker::GetMainComponent
+// and GetCrossComponent are cleaner; but in cases where we simply have
+// two separate expressions for width and height (which may be expensive to
+// evaluate), these macros will ensure that only the expression for the correct
+// axis gets evaluated.
+#define GET_MAIN_COMPONENT(axisTracker_, width_, height_)  \
+  IsAxisHorizontal((axisTracker_).GetMainAxis()) ? (width_) : (height_)
+
+#define GET_CROSS_COMPONENT(axisTracker_, width_, height_)  \
+  IsAxisHorizontal((axisTracker_).GetCrossAxis()) ? (width_) : (height_)
+
+// Encapsulates our flex container's main & cross axes.
+class MOZ_STACK_CLASS nsFlexContainerFrame::FlexboxAxisTracker {
+public:
+  FlexboxAxisTracker(nsFlexContainerFrame* aFlexContainerFrame);
+
+  // Accessors:
+  AxisOrientationType GetMainAxis() const  { return mMainAxis;  }
+  AxisOrientationType GetCrossAxis() const { return mCrossAxis; }
+
+  nscoord GetMainComponent(const nsSize& aSize) const {
+    return GET_MAIN_COMPONENT(*this, aSize.width, aSize.height);
+  }
+  int32_t GetMainComponent(const nsIntSize& aIntSize) const {
+    return GET_MAIN_COMPONENT(*this, aIntSize.width, aIntSize.height);
+  }
+
+  nscoord GetCrossComponent(const nsSize& aSize) const {
+    return GET_CROSS_COMPONENT(*this, aSize.width, aSize.height);
+  }
+  int32_t GetCrossComponent(const nsIntSize& aIntSize) const {
+    return GET_CROSS_COMPONENT(*this, aIntSize.width, aIntSize.height);
+  }
+
+  nscoord GetMarginSizeInMainAxis(const nsMargin& aMargin) const {
+    return IsAxisHorizontal(mMainAxis) ?
+      aMargin.LeftRight() :
+      aMargin.TopBottom();
+  }
+  nscoord GetMarginSizeInCrossAxis(const nsMargin& aMargin) const {
+    return IsAxisHorizontal(mCrossAxis) ?
+      aMargin.LeftRight() :
+      aMargin.TopBottom();
+  }
+
+  /**
+   * Converts a logical point into a "physical" x,y point.
+   *
+   * In the simplest case where the main-axis is left-to-right and the
+   * cross-axis is top-to-bottom, this just returns
+   * nsPoint(aMainPosn, aCrossPosn).
+   *
+   *  @arg aMainPosn  The main-axis position -- i.e an offset from the
+   *                  main-start edge of the container's content box.
+   *  @arg aCrossPosn The cross-axis position -- i.e an offset from the
+   *                  cross-start edge of the container's content box.
+   *  @return A nsPoint representing the same position (in coordinates
+   *          relative to the container's content box).
+   */
+  nsPoint PhysicalPointFromLogicalPoint(nscoord aMainPosn,
+                                        nscoord aCrossPosn,
+                                        nscoord aContainerMainSize,
+                                        nscoord aContainerCrossSize) const {
+    nscoord physicalPosnInMainAxis =
+      PhysicalPosFromLogicalPos(aMainPosn, aContainerMainSize, mMainAxis);
+    nscoord physicalPosnInCrossAxis =
+      PhysicalPosFromLogicalPos(aCrossPosn, aContainerCrossSize, mCrossAxis);
+
+    return IsAxisHorizontal(mMainAxis) ?
+      nsPoint(physicalPosnInMainAxis, physicalPosnInCrossAxis) :
+      nsPoint(physicalPosnInCrossAxis, physicalPosnInMainAxis);
+  }
+  nsSize PhysicalSizeFromLogicalSizes(nscoord aMainSize,
+                                      nscoord aCrossSize) const {
+    return IsAxisHorizontal(mMainAxis) ?
+      nsSize(aMainSize, aCrossSize) :
+      nsSize(aCrossSize, aMainSize);
+  }
+
+  // Are my axes reversed with respect to what the author asked for?
+  // (We may reverse the axes in the FlexboxAxisTracker constructor and set
+  // this flag, to avoid reflowing our children in bottom-to-top order.)
+  bool AreAxesInternallyReversed() const
+  {
+    return mAreAxesInternallyReversed;
+  }
+
+private:
+  AxisOrientationType mMainAxis;
+  AxisOrientationType mCrossAxis;
+  bool mAreAxesInternallyReversed;
+};
+
+/**
+ * Represents a flex item.
+ * Includes the various pieces of input that the Flexbox Layout Algorithm uses
+ * to resolve a flexible width.
+ */
+class nsFlexContainerFrame::FlexItem : public LinkedListElement<FlexItem>
+{
+public:
+  // Normal constructor:
+  FlexItem(nsIFrame* aChildFrame,
+           float aFlexGrow, float aFlexShrink, nscoord aMainBaseSize,
+           nscoord aMainMinSize, nscoord aMainMaxSize,
+           nscoord aCrossMinSize, nscoord aCrossMaxSize,
+           nsMargin aMargin, nsMargin aBorderPadding,
+           const FlexboxAxisTracker& aAxisTracker);
+
+  // Simplified constructor, to be used only for generating "struts":
+  FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize);
+
+  // Accessors
+  nsIFrame* Frame() const          { return mFrame; }
+  nscoord GetFlexBaseSize() const  { return mFlexBaseSize; }
+
+  nscoord GetMainMinSize() const   { return mMainMinSize; }
+  nscoord GetMainMaxSize() const   { return mMainMaxSize; }
+
+  // Note: These return the main-axis position and size of our *content box*.
+  nscoord GetMainSize() const      { return mMainSize; }
+  nscoord GetMainPosition() const  { return mMainPosn; }
+
+  nscoord GetCrossMinSize() const  { return mCrossMinSize; }
+  nscoord GetCrossMaxSize() const  { return mCrossMaxSize; }
+
+  // Note: These return the cross-axis position and size of our *content box*.
+  nscoord GetCrossSize() const     { return mCrossSize;  }
+  nscoord GetCrossPosition() const { return mCrossPosn; }
+
+  // Convenience methods to compute the main & cross size of our *margin-box*.
+  // The caller is responsible for telling us the right axis, so that we can
+  // pull out the appropriate components of our margin/border/padding structs.
+  nscoord GetOuterMainSize(AxisOrientationType aMainAxis) const
+  {
+    return mMainSize + GetMarginBorderPaddingSizeInAxis(aMainAxis);
+  }
+
+  nscoord GetOuterCrossSize(AxisOrientationType aCrossAxis) const
+  {
+    return mCrossSize + GetMarginBorderPaddingSizeInAxis(aCrossAxis);
+  }
+
+  // Returns the distance between this FlexItem's baseline and the cross-start
+  // edge of its margin-box. Used in baseline alignment.
+  // (This function needs to be told what cross axis is & which edge we're
+  // measuring the baseline from, so that it can look up the appropriate
+  // components from mMargin.)
+  nscoord GetBaselineOffsetFromOuterCrossEdge(AxisOrientationType aCrossAxis,
+                                              AxisEdgeType aEdge) const;
+
+  float GetShareOfFlexWeightSoFar() const { return mShareOfFlexWeightSoFar; }
+
+  bool IsFrozen() const            { return mIsFrozen; }
+
+  bool HadMinViolation() const     { return mHadMinViolation; }
+  bool HadMaxViolation() const     { return mHadMaxViolation; }
+
+  // Indicates whether this item received a preliminary "measuring" reflow
+  // before its actual reflow.
+  bool HadMeasuringReflow() const  { return mHadMeasuringReflow; }
+
+  // Indicates whether this item's cross-size has been stretched (from having
+  // "align-self: stretch" with an auto cross-size and no auto margins in the
+  // cross axis).
+  bool IsStretched() const         { return mIsStretched; }
+
+  // Indicates whether this item is a "strut" left behind by an element with
+  // visibility:collapse.
+  bool IsStrut() const             { return mIsStrut; }
+
+  uint8_t GetAlignSelf() const     { return mAlignSelf; }
+
+  // Returns the flex weight that we should use in the "resolving flexible
+  // lengths" algorithm.  If we're using flex grow, we just return that;
+  // otherwise, we use the "scaled flex shrink weight" (scaled by our flex
+  // base size, so that when both large and small items are shrinking,
+  // the large items shrink more).
+  float GetFlexWeightToUse(bool aIsUsingFlexGrow)
+  {
+    if (IsFrozen()) {
+      return 0.0f;
+    }
+
+    if (aIsUsingFlexGrow) {
+      return mFlexGrow;
+    }
+
+    // We're using flex-shrink --> return mFlexShrink * mFlexBaseSize
+    if (mFlexBaseSize == 0) {
+      // Special-case for mFlexBaseSize == 0 -- we have no room to shrink, so
+      // regardless of mFlexShrink, we should just return 0.
+      // (This is really a special-case for when mFlexShrink is infinity, to
+      // avoid performing mFlexShrink * mFlexBaseSize = inf * 0 = undefined.)
+      return 0.0f;
+    }
+    return mFlexShrink * mFlexBaseSize;
+  }
+
+  // Getters for margin:
+  // ===================
+  const nsMargin& GetMargin() const { return mMargin; }
+
+  // Returns the margin component for a given mozilla::css::Side
+  nscoord GetMarginComponentForSide(Side aSide) const
+  { return MarginComponentForSide(mMargin, aSide); }
+
+  // Returns the total space occupied by this item's margins in the given axis
+  nscoord GetMarginSizeInAxis(AxisOrientationType aAxis) const
+  {
+    Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
+    Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
+    return GetMarginComponentForSide(startSide) +
+      GetMarginComponentForSide(endSide);
+  }
+
+  // Getters for border/padding
+  // ==========================
+  const nsMargin& GetBorderPadding() const { return mBorderPadding; }
+
+  // Returns the border+padding component for a given mozilla::css::Side
+  nscoord GetBorderPaddingComponentForSide(Side aSide) const
+  { return MarginComponentForSide(mBorderPadding, aSide); }
+
+  // Returns the total space occupied by this item's borders and padding in
+  // the given axis
+  nscoord GetBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
+  {
+    Side startSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_Start];
+    Side endSide = kAxisOrientationToSidesMap[aAxis][eAxisEdge_End];
+    return GetBorderPaddingComponentForSide(startSide) +
+      GetBorderPaddingComponentForSide(endSide);
+  }
+
+  // Getter for combined margin/border/padding
+  // =========================================
+  // Returns the total space occupied by this item's margins, borders and
+  // padding in the given axis
+  nscoord GetMarginBorderPaddingSizeInAxis(AxisOrientationType aAxis) const
+  {
+    return GetMarginSizeInAxis(aAxis) + GetBorderPaddingSizeInAxis(aAxis);
+  }
+
+  // Setters
+  // =======
+
+  // This sets our flex base size, and then updates the main size to the
+  // base size clamped to our main-axis [min,max] constraints.
+  void SetFlexBaseSizeAndMainSize(nscoord aNewFlexBaseSize)
+  {
+    MOZ_ASSERT(!mIsFrozen || mFlexBaseSize == NS_INTRINSICSIZE,
+               "flex base size shouldn't change after we're frozen "
+               "(unless we're just resolving an intrinsic size)");
+    mFlexBaseSize = aNewFlexBaseSize;
+
+    // Before we've resolved flexible lengths, we keep mMainSize set to
+    // the 'hypothetical main size', which is the flex base size, clamped
+    // to the [min,max] range:
+    mMainSize = NS_CSS_MINMAX(mFlexBaseSize, mMainMinSize, mMainMaxSize);
+  }
+
+  // Setters used while we're resolving flexible lengths
+  // ---------------------------------------------------
+
+  // Sets the main-size of our flex item's content-box.
+  void SetMainSize(nscoord aNewMainSize)
+  {
+    MOZ_ASSERT(!mIsFrozen, "main size shouldn't change after we're frozen");
+    mMainSize = aNewMainSize;
+  }
+
+  void SetShareOfFlexWeightSoFar(float aNewShare)
+  {
+    MOZ_ASSERT(!mIsFrozen || aNewShare == 0.0f,
+               "shouldn't be giving this item any share of the weight "
+               "after it's frozen");
+    mShareOfFlexWeightSoFar = aNewShare;
+  }
+
+  void Freeze() { mIsFrozen = true; }
+
+  void SetHadMinViolation()
+  {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMinViolation = true;
+  }
+  void SetHadMaxViolation()
+  {
+    MOZ_ASSERT(!mIsFrozen,
+               "shouldn't be changing main size & having violations "
+               "after we're frozen");
+    mHadMaxViolation = true;
+  }
+  void ClearViolationFlags()
+  { mHadMinViolation = mHadMaxViolation = false; }
+
+  // Setters for values that are determined after we've resolved our main size
+  // -------------------------------------------------------------------------
+
+  // Sets the main-axis position of our flex item's content-box.
+  // (This is the distance between the main-start edge of the flex container
+  // and the main-start edge of the flex item's content-box.)
+  void SetMainPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mMainPosn  = aPosn;
+  }
+
+  // Sets the cross-size of our flex item's content-box.
+  void SetCrossSize(nscoord aCrossSize) {
+    MOZ_ASSERT(!mIsStretched,
+               "Cross size shouldn't be modified after it's been stretched");
+    mCrossSize = aCrossSize;
+  }
+
+  // Sets the cross-axis position of our flex item's content-box.
+  // (This is the distance between the cross-start edge of the flex container
+  // and the cross-start edge of the flex item.)
+  void SetCrossPosition(nscoord aPosn) {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mCrossPosn = aPosn;
+  }
+
+  void SetAscent(nscoord aAscent) {
+    mAscent = aAscent;
+  }
+
+  void SetHadMeasuringReflow() {
+    mHadMeasuringReflow = true;
+  }
+
+  void SetIsStretched() {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    mIsStretched = true;
+  }
+
+  // Setter for margin components (for resolving "auto" margins)
+  void SetMarginComponentForSide(Side aSide, nscoord aLength)
+  {
+    MOZ_ASSERT(mIsFrozen, "main size should be resolved before this");
+    MarginComponentForSide(mMargin, aSide) = aLength;
+  }
+
+  void ResolveStretchedCrossSize(nscoord aLineCrossSize,
+                                 const FlexboxAxisTracker& aAxisTracker);
+
+  uint32_t GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const;
+
+protected:
+  // Our frame:
+  nsIFrame* const mFrame;
+
+  // Values that we already know in constructor: (and are hence mostly 'const')
+  const float mFlexGrow;
+  const float mFlexShrink;
+
+  const nsMargin mBorderPadding;
+  nsMargin mMargin; // non-const because we need to resolve auto margins
+
+  nscoord mFlexBaseSize;
+
+  const nscoord mMainMinSize;
+  const nscoord mMainMaxSize;
+  const nscoord mCrossMinSize;
+  const nscoord mCrossMaxSize;
+
+  // Values that we compute after constructor:
+  nscoord mMainSize;
+  nscoord mMainPosn;
+  nscoord mCrossSize;
+  nscoord mCrossPosn;
+  nscoord mAscent;
+
+  // Temporary state, while we're resolving flexible widths (for our main size)
+  // XXXdholbert To save space, we could use a union to make these variables
+  // overlay the same memory as some other member vars that aren't touched
+  // until after main-size has been resolved. In particular, these could share
+  // memory with mMainPosn through mAscent, and mIsStretched.
+  float mShareOfFlexWeightSoFar;
+  bool mIsFrozen;
+  bool mHadMinViolation;
+  bool mHadMaxViolation;
+
+  // Misc:
+  bool mHadMeasuringReflow; // Did this item get a preliminary reflow,
+                            // to measure its desired height?
+  bool mIsStretched; // See IsStretched() documentation
+  bool mIsStrut;     // Is this item a "strut" left behind by an element
+                     // with visibility:collapse?
+  uint8_t mAlignSelf; // My "align-self" computed value (with "auto"
+                      // swapped out for parent"s "align-items" value,
+                      // in our constructor).
+};
+
+/**
+ * Represents a single flex line in a flex container.
+ * Manages a linked list of the FlexItems that are in the line.
+ */
+class nsFlexContainerFrame::FlexLine : public LinkedListElement<FlexLine>
+{
+public:
+  FlexLine()
+  : mNumItems(0),
+    mTotalInnerHypotheticalMainSize(0),
+    mTotalOuterHypotheticalMainSize(0),
+    mLineCrossSize(0),
+    mBaselineOffset(nscoord_MIN)
+  {}
+
+  // Returns the sum of our FlexItems' outer hypothetical main sizes.
+  // ("outer" = margin-box, and "hypothetical" = before flexing)
+  nscoord GetTotalOuterHypotheticalMainSize() const {
+    return mTotalOuterHypotheticalMainSize;
+  }
+
+  // Accessors for our FlexItems & information about them:
+  FlexItem* GetFirstItem()
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getFirst();
+  }
+
+  const FlexItem* GetFirstItem() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getFirst();
+  }
+
+  bool IsEmpty() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.isEmpty();
+  }
+
+  uint32_t NumItems() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mNumItems;
+  }
+
+  // Adds the given FlexItem to our list of items (at the front or back
+  // depending on aShouldInsertAtFront), and adds its hypothetical
+  // outer & inner main sizes to our totals. Use this method instead of
+  // directly modifying the item list, so that our bookkeeping remains correct.
+  void AddItem(FlexItem* aItem,
+               bool aShouldInsertAtFront,
+               nscoord aItemInnerHypotheticalMainSize,
+               nscoord aItemOuterHypotheticalMainSize)
+  {
+    if (aShouldInsertAtFront) {
+      mItems.insertFront(aItem);
+    } else {
+      mItems.insertBack(aItem);
+    }
+    mNumItems++;
+    mTotalInnerHypotheticalMainSize += aItemInnerHypotheticalMainSize;
+    mTotalOuterHypotheticalMainSize += aItemOuterHypotheticalMainSize;
+  }
+
+  // Computes the cross-size and baseline position of this FlexLine, based on
+  // its FlexItems.
+  void ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker);
+
+  // Returns the cross-size of this line.
+  nscoord GetLineCrossSize() const { return mLineCrossSize; }
+
+  // Setter for line cross-size -- needed for cases where the flex container
+  // imposes a cross-size on the line. (e.g. for single-line flexbox, or for
+  // multi-line flexbox with 'align-content: stretch')
+  void SetLineCrossSize(nscoord aLineCrossSize) {
+    mLineCrossSize = aLineCrossSize;
+  }
+
+  /**
+   * Returns the offset within this line where any baseline-aligned FlexItems
+   * should place their baseline. Usually, this represents a distance from the
+   * line's cross-start edge, but if we're internally reversing the axes (see
+   * AreAxesInternallyReversed()), this instead represents the distance from
+   * its cross-end edge.
+   *
+   * If there are no baseline-aligned FlexItems, returns nscoord_MIN.
+   */
+  nscoord GetBaselineOffset() const {
+    return mBaselineOffset;
+  }
+
+  // Runs the "Resolving Flexible Lengths" algorithm from section 9.7 of the
+  // CSS flexbox spec to distribute aFlexContainerMainSize among our flex items.
+  void ResolveFlexibleLengths(nscoord aFlexContainerMainSize);
+
+  void PositionItemsInMainAxis(uint8_t aJustifyContent,
+                               nscoord aContentBoxMainSize,
+                               const FlexboxAxisTracker& aAxisTracker);
+
+  void PositionItemsInCrossAxis(nscoord aLineStartPosition,
+                                const FlexboxAxisTracker& aAxisTracker);
+
+  friend class AutoFlexLineListClearer; // (needs access to mItems)
+
+private:
+  // Helper for ResolveFlexibleLengths():
+  void FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                       bool aIsFinalIteration);
+
+  LinkedList<FlexItem> mItems; // Linked list of this line's flex items.
+
+  uint32_t mNumItems; // Number of FlexItems in this line (in |mItems|).
+                      // (Shouldn't change after GenerateFlexLines finishes
+                      // with this line -- at least, not until we add support
+                      // for splitting lines across continuations. Then we can
+                      // update this count carefully.)
+
+  nscoord mTotalInnerHypotheticalMainSize;
+  nscoord mTotalOuterHypotheticalMainSize;
+  nscoord mLineCrossSize;
+  nscoord mBaselineOffset;
+};
+
+// Information about a strut left behind by a FlexItem that's been collapsed
+// using "visibility:collapse".
+struct nsFlexContainerFrame::StrutInfo {
+  StrutInfo(uint32_t aItemIdx, nscoord aStrutCrossSize)
+    : mItemIdx(aItemIdx),
+      mStrutCrossSize(aStrutCrossSize)
+  {
+  }
+
+  uint32_t mItemIdx;      // Index in the child list.
+  nscoord mStrutCrossSize; // The cross-size of this strut.
+};
+
+static void
+BuildStrutInfoFromCollapsedItems(const FlexLine* aFirstLine,
+                                 nsTArray<StrutInfo>& aStruts)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+  MOZ_ASSERT(aStruts.IsEmpty(),
+             "We should only build up StrutInfo once per reflow, so "
+             "aStruts should be empty when this is called");
+
+  uint32_t itemIdxInContainer = 0;
+  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    for (const FlexItem* item = line->GetFirstItem(); item;
+         item = item->getNext()) {
+      if (NS_STYLE_VISIBILITY_COLLAPSE ==
+          item->Frame()->StyleVisibility()->mVisible) {
+        // Note the cross size of the line as the item's strut size.
+        aStruts.AppendElement(StrutInfo(itemIdxInContainer,
+                                        line->GetLineCrossSize()));
+      }
+      itemIdxInContainer++;
+    }
+  }
+}
+
+// Helper-function to find the first non-anonymous-box descendent of aFrame.
+static nsIFrame*
+GetFirstNonAnonBoxDescendant(nsIFrame* aFrame)
+{
+  while (aFrame) {
+    nsIAtom* pseudoTag = aFrame->StyleContext()->GetPseudo();
+
+    // If aFrame isn't an anonymous container, then it'll do.
+    if (!pseudoTag ||                                 // No pseudotag.
+        !nsCSSAnonBoxes::IsAnonBox(pseudoTag) ||      // Pseudotag isn't anon.
+        pseudoTag == nsCSSAnonBoxes::mozNonElement) { // Text, not a container.
+      break;
+    }
+
+    // Otherwise, descend to its first child and repeat.
+
+    // SPECIAL CASE: if we're dealing with an anonymous table, then it might
+    // be wrapping something non-anonymous in its caption or col-group lists
+    // (instead of its principal child list), so we have to look there.
+    // (Note: For anonymous tables that have a non-anon cell *and* a non-anon
+    // column, we'll always return the column. This is fine; we're really just
+    // looking for a handle to *anything* with a meaningful content node inside
+    // the table, for use in DOM comparisons to things outside of the table.)
+    if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableOuterFrame)) {
+      nsIFrame* captionDescendant =
+        GetFirstNonAnonBoxDescendant(aFrame->GetFirstChild(kCaptionList));
+      if (captionDescendant) {
+        return captionDescendant;
+      }
+    } else if (MOZ_UNLIKELY(aFrame->GetType() == nsGkAtoms::tableFrame)) {
+      nsIFrame* colgroupDescendant =
+        GetFirstNonAnonBoxDescendant(aFrame->GetFirstChild(kColGroupList));
+      if (colgroupDescendant) {
+        return colgroupDescendant;
+      }
+    }
+
+    // USUAL CASE: Descend to the first child in principal list.
+    aFrame = aFrame->GetFirstPrincipalChild();
+  }
+  return aFrame;
+}
+
+/**
+ * Sorting helper-function that compares two frames' "order" property-values,
+ * and if they're equal, compares the DOM positions of their corresponding
+ * content nodes. Returns true if aFrame1 is "less than or equal to" aFrame2
+ * according to this comparison.
+ *
+ * Note: This can't be a static function, because we need to pass it as a
+ * template argument. (Only functions with external linkage can be passed as
+ * template arguments.)
+ *
+ * @return true if the computed "order" property of aFrame1 is less than that
+ *         of aFrame2, or if the computed "order" values are equal and aFrame1's
+ *         corresponding DOM node is earlier than aFrame2's in the DOM tree.
+ *         Otherwise, returns false.
+ */
+bool
+IsOrderLEQWithDOMFallback(nsIFrame* aFrame1,
+                          nsIFrame* aFrame2)
+{
+  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
+             "this method only intended for comparing flex items");
+
+  if (aFrame1 == aFrame2) {
+    // Anything is trivially LEQ itself, so we return "true" here... but it's
+    // probably bad if we end up actually needing this, so let's assert.
+    NS_ERROR("Why are we checking if a frame is LEQ itself?");
+    return true;
+  }
+
+  // If we've got a placeholder frame, use its out-of-flow frame's 'order' val.
+  {
+    nsIFrame* aRealFrame1 = nsPlaceholderFrame::GetRealFrameFor(aFrame1);
+    nsIFrame* aRealFrame2 = nsPlaceholderFrame::GetRealFrameFor(aFrame2);
+
+    int32_t order1 = aRealFrame1->StylePosition()->mOrder;
+    int32_t order2 = aRealFrame2->StylePosition()->mOrder;
+
+    if (order1 != order2) {
+      return order1 < order2;
+    }
+  }
+
+  // The "order" values are equal, so we need to fall back on DOM comparison.
+  // For that, we need to dig through any anonymous box wrapper frames to find
+  // the actual frame that corresponds to our child content.
+  aFrame1 = GetFirstNonAnonBoxDescendant(aFrame1);
+  aFrame2 = GetFirstNonAnonBoxDescendant(aFrame2);
+  MOZ_ASSERT(aFrame1 && aFrame2,
+             "why do we have an anonymous box without any "
+             "non-anonymous descendants?");
+
+
+  // Special case:
+  // If either frame is for generated content from ::before or ::after, then
+  // we can't use nsContentUtils::PositionIsBefore(), since that method won't
+  // recognize generated content as being an actual sibling of other nodes.
+  // We know where ::before and ::after nodes *effectively* insert in the DOM
+  // tree, though (at the beginning & end), so we can just special-case them.
+  nsIAtom* pseudo1 = aFrame1->StyleContext()->GetPseudo();
+  nsIAtom* pseudo2 = aFrame2->StyleContext()->GetPseudo();
+  if (pseudo1 == nsCSSPseudoElements::before ||
+      pseudo2 == nsCSSPseudoElements::after) {
+    // frame1 is ::before and/or frame2 is ::after => frame1 is LEQ frame2.
+    return true;
+  }
+  if (pseudo1 == nsCSSPseudoElements::after ||
+      pseudo2 == nsCSSPseudoElements::before) {
+    // frame1 is ::after and/or frame2 is ::before => frame1 is not LEQ frame2.
+    return false;
+  }
+
+  // Usual case: Compare DOM position.
+  nsIContent* content1 = aFrame1->GetContent();
+  nsIContent* content2 = aFrame2->GetContent();
+  MOZ_ASSERT(content1 != content2,
+             "Two different flex items are using the same nsIContent node for "
+             "comparison, so we may be sorting them in an arbitrary order");
+
+  return nsContentUtils::PositionIsBefore(content1, content2);
+}
+
+/**
+ * Sorting helper-function that compares two frames' "order" property-values.
+ * Returns true if aFrame1 is "less than or equal to" aFrame2 according to this
+ * comparison.
+ *
+ * Note: This can't be a static function, because we need to pass it as a
+ * template argument. (Only functions with external linkage can be passed as
+ * template arguments.)
+ *
+ * @return true if the computed "order" property of aFrame1 is less than or
+ *         equal to that of aFrame2.  Otherwise, returns false.
+ */
+bool
+IsOrderLEQ(nsIFrame* aFrame1,
+           nsIFrame* aFrame2)
+{
+  MOZ_ASSERT(aFrame1->IsFlexItem() && aFrame2->IsFlexItem(),
+             "this method only intended for comparing flex items");
+
+  // If we've got a placeholder frame, use its out-of-flow frame's 'order' val.
+  nsIFrame* aRealFrame1 = nsPlaceholderFrame::GetRealFrameFor(aFrame1);
+  nsIFrame* aRealFrame2 = nsPlaceholderFrame::GetRealFrameFor(aFrame2);
+
+  int32_t order1 = aRealFrame1->StylePosition()->mOrder;
+  int32_t order2 = aRealFrame2->StylePosition()->mOrder;
+
+  return order1 <= order2;
+}
+
+bool
+nsFlexContainerFrame::IsHorizontal()
+{
+  const FlexboxAxisTracker axisTracker(this);
+  return IsAxisHorizontal(axisTracker.GetMainAxis());
+}
+
+FlexItem*
+nsFlexContainerFrame::GenerateFlexItemForChild(
+  nsPresContext* aPresContext,
+  nsIFrame*      aChildFrame,
+  const nsHTMLReflowState& aParentReflowState,
+  const FlexboxAxisTracker& aAxisTracker)
+{
+  // Create temporary reflow state just for sizing -- to get hypothetical
+  // main-size and the computed values of min / max main-size property.
+  // (This reflow state will _not_ be used for reflow.)
+  nsHTMLReflowState childRS(aPresContext, aParentReflowState, aChildFrame,
+                            nsSize(aParentReflowState.ComputedWidth(),
+                                   aParentReflowState.ComputedHeight()));
+
+  // FLEX GROW & SHRINK WEIGHTS
+  // --------------------------
+  const nsStylePosition* stylePos = aChildFrame->StylePosition();
+  float flexGrow   = stylePos->mFlexGrow;
+  float flexShrink = stylePos->mFlexShrink;
+
+  // MAIN SIZES (flex base size, min/max size)
+  // -----------------------------------------
+  nscoord flexBaseSize = GET_MAIN_COMPONENT(aAxisTracker,
+                                            childRS.ComputedWidth(),
+                                            childRS.ComputedHeight());
+  nscoord mainMinSize = GET_MAIN_COMPONENT(aAxisTracker,
+                                           childRS.ComputedMinWidth(),
+                                           childRS.ComputedMinHeight());
+  nscoord mainMaxSize = GET_MAIN_COMPONENT(aAxisTracker,
+                                           childRS.ComputedMaxWidth(),
+                                           childRS.ComputedMaxHeight());
+  // This is enforced by the nsHTMLReflowState where these values come from:
+  MOZ_ASSERT(mainMinSize <= mainMaxSize, "min size is larger than max size");
+
+  // CROSS MIN/MAX SIZE
+  // ------------------
+
+  nscoord crossMinSize = GET_CROSS_COMPONENT(aAxisTracker,
+                                             childRS.ComputedMinWidth(),
+                                             childRS.ComputedMinHeight());
+  nscoord crossMaxSize = GET_CROSS_COMPONENT(aAxisTracker,
+                                             childRS.ComputedMaxWidth(),
+                                             childRS.ComputedMaxHeight());
+
+  // SPECIAL-CASE FOR WIDGET-IMPOSED SIZES
+  // Check if we're a themed widget, in which case we might have a minimum
+  // main & cross size imposed by our widget (which we can't go below), or
+  // (more severe) our widget might have only a single valid size.
+  bool isFixedSizeWidget = false;
+  const nsStyleDisplay* disp = aChildFrame->StyleDisplay();
+  if (aChildFrame->IsThemed(disp)) {
+    nsIntSize widgetMinSize(0, 0);
+    bool canOverride = true;
+    aPresContext->GetTheme()->
+      GetMinimumWidgetSize(childRS.rendContext, aChildFrame,
+                           disp->mAppearance,
+                           &widgetMinSize, &canOverride);
+
+    nscoord widgetMainMinSize =
+      aPresContext->DevPixelsToAppUnits(
+        aAxisTracker.GetMainComponent(widgetMinSize));
+    nscoord widgetCrossMinSize =
+      aPresContext->DevPixelsToAppUnits(
+        aAxisTracker.GetCrossComponent(widgetMinSize));
+
+    // GMWS() returns border-box. We need content-box, so subtract
+    // borderPadding (but don't let that push our min sizes below 0).
+    nsMargin& bp = childRS.ComputedPhysicalBorderPadding();
+    widgetMainMinSize = std::max(widgetMainMinSize -
+                                 aAxisTracker.GetMarginSizeInMainAxis(bp), 0);
+    widgetCrossMinSize = std::max(widgetCrossMinSize -
+                                  aAxisTracker.GetMarginSizeInCrossAxis(bp), 0);
+
+    if (!canOverride) {
+      // Fixed-size widget: freeze our main-size at the widget's mandated size.
+      // (Set min and max main-sizes to that size, too, to keep us from
+      // clamping to any other size later on.)
+      flexBaseSize = mainMinSize = mainMaxSize = widgetMainMinSize;
+      crossMinSize = crossMaxSize = widgetCrossMinSize;
+      isFixedSizeWidget = true;
+    } else {
+      // Variable-size widget: ensure our min/max sizes are at least as large
+      // as the widget's mandated minimum size, so we don't flex below that.
+      mainMinSize = std::max(mainMinSize, widgetMainMinSize);
+      mainMaxSize = std::max(mainMaxSize, widgetMainMinSize);
+
+      crossMinSize = std::max(crossMinSize, widgetCrossMinSize);
+      crossMaxSize = std::max(crossMaxSize, widgetCrossMinSize);
+    }
+  }
+
+  // Construct the flex item!
+  FlexItem* item = new FlexItem(aChildFrame,
+                                flexGrow, flexShrink, flexBaseSize,
+                                mainMinSize, mainMaxSize,
+                                crossMinSize, crossMaxSize,
+                                childRS.ComputedPhysicalMargin(),
+                                childRS.ComputedPhysicalBorderPadding(),
+                                aAxisTracker);
+
+  // If we're inflexible, we can just freeze to our hypothetical main-size
+  // up-front. Similarly, if we're a fixed-size widget, we only have one
+  // valid size, so we freeze to keep ourselves from flexing.
+  if (isFixedSizeWidget || (flexGrow == 0.0f && flexShrink == 0.0f)) {
+    item->Freeze();
+  }
+
+  return item;
+}
+
+nsresult
+nsFlexContainerFrame::
+  ResolveFlexItemMaxContentSizing(nsPresContext* aPresContext,
+                                  FlexItem& aFlexItem,
+                                  const nsHTMLReflowState& aParentReflowState,
+                                  const FlexboxAxisTracker& aAxisTracker)
+{
+  if (IsAxisHorizontal(aAxisTracker.GetMainAxis())) {
+    // Nothing to do -- this function is only for measuring flex items
+    // in a vertical flex container.
+    return NS_OK;
+  }
+
+  if (NS_AUTOHEIGHT != aFlexItem.GetFlexBaseSize()) {
+    // Nothing to do; this function's only relevant for flex items
+    // with a base size of "auto" (or equivalent).
+    // XXXdholbert If & when we handle "min-height: min-content" for flex items,
+    // we'll want to resolve that in this function, too.
+    return NS_OK;
+  }
+
+  // If we get here, we're vertical and our main size ended up being
+  // unconstrained. We need to use our "max-content" height, which is what we
+  // get from reflowing into our available width.
+  // Note: This has to come *after* we construct the FlexItem, since we
+  // invoke at least one convenience method (ResolveStretchedCrossSize) which
+  // requires a FlexItem.
+
+  // Give the item a special reflow with "mIsFlexContainerMeasuringHeight"
+  // set.  This tells it to behave as if it had "height: auto", regardless
+  // of what the "height" property is actually set to.
+  nsHTMLReflowState
+    childRSForMeasuringHeight(aPresContext, aParentReflowState,
+                              aFlexItem.Frame(),
+                              nsSize(aParentReflowState.ComputedWidth(),
+                                     NS_UNCONSTRAINEDSIZE),
+                              -1, -1, nsHTMLReflowState::CALLER_WILL_INIT);
+  childRSForMeasuringHeight.mFlags.mIsFlexContainerMeasuringHeight = true;
+  childRSForMeasuringHeight.Init(aPresContext);
+
+  aFlexItem.ResolveStretchedCrossSize(aParentReflowState.ComputedWidth(),
+                                      aAxisTracker);
+  if (aFlexItem.IsStretched()) {
+    childRSForMeasuringHeight.SetComputedWidth(aFlexItem.GetCrossSize());
+    childRSForMeasuringHeight.mFlags.mHResize = true;
+  }
+
+  // If this item is flexible (vertically), then we assume that the
+  // computed-height we're reflowing with now could be different
+  // from the one we'll use for this flex item's "actual" reflow later on.
+  // In that case, we need to be sure the flex item treats this as a
+  // vertical resize, even though none of its ancestors are necessarily
+  // being vertically resized.
+  // (Note: We don't have to do this for width, because InitResizeFlags
+  // will always turn on mHResize on when it sees that the computed width
+  // is different from current width, and that's all we need.)
+  if (!aFlexItem.IsFrozen()) {  // Are we flexible?
+    childRSForMeasuringHeight.mFlags.mVResize = true;
+  }
+
+  nsHTMLReflowMetrics childDesiredSize(childRSForMeasuringHeight);
+  nsReflowStatus childReflowStatus;
+  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
+  nsresult rv = ReflowChild(aFlexItem.Frame(), aPresContext,
+                            childDesiredSize, childRSForMeasuringHeight,
+                            0, 0, flags, childReflowStatus);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+             "We gave flex item unconstrained available height, so it "
+             "should be complete");
+
+  rv = FinishReflowChild(aFlexItem.Frame(), aPresContext,
+                         childDesiredSize, &childRSForMeasuringHeight,
+                         0, 0, flags);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Subtract border/padding in vertical axis, to get _just_
+  // the effective computed value of the "height" property.
+  nscoord childDesiredHeight = childDesiredSize.Height() -
+    childRSForMeasuringHeight.ComputedPhysicalBorderPadding().TopBottom();
+  childDesiredHeight = std::max(0, childDesiredHeight);
+
+  aFlexItem.SetFlexBaseSizeAndMainSize(childDesiredHeight);
+  aFlexItem.SetHadMeasuringReflow();
+
+  return NS_OK;
+}
+
+FlexItem::FlexItem(nsIFrame* aChildFrame,
+                   float aFlexGrow, float aFlexShrink, nscoord aFlexBaseSize,
+                   nscoord aMainMinSize,  nscoord aMainMaxSize,
+                   nscoord aCrossMinSize, nscoord aCrossMaxSize,
+                   nsMargin aMargin, nsMargin aBorderPadding,
+                   const FlexboxAxisTracker& aAxisTracker)
+  : mFrame(aChildFrame),
+    mFlexGrow(aFlexGrow),
+    mFlexShrink(aFlexShrink),
+    mBorderPadding(aBorderPadding),
+    mMargin(aMargin),
+    mMainMinSize(aMainMinSize),
+    mMainMaxSize(aMainMaxSize),
+    mCrossMinSize(aCrossMinSize),
+    mCrossMaxSize(aCrossMaxSize),
+    mMainPosn(0),
+    mCrossSize(0),
+    mCrossPosn(0),
+    mAscent(0),
+    mShareOfFlexWeightSoFar(0.0f),
+    mIsFrozen(false),
+    mHadMinViolation(false),
+    mHadMaxViolation(false),
+    mHadMeasuringReflow(false),
+    mIsStretched(false),
+    mIsStrut(false),
+    mAlignSelf(aChildFrame->StylePosition()->mAlignSelf)
+{
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+
+  SetFlexBaseSizeAndMainSize(aFlexBaseSize);
+
+  // Assert that any "auto" margin components are set to 0.
+  // (We'll resolve them later; until then, we want to treat them as 0-sized.)
+#ifdef DEBUG
+  {
+    const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
+    NS_FOR_CSS_SIDES(side) {
+      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+        MOZ_ASSERT(GetMarginComponentForSide(side) == 0,
+                   "Someone else tried to resolve our auto margin");
+      }
+    }
+  }
+#endif // DEBUG
+
+  // Resolve "align-self: auto" to parent's "align-items" value.
+  if (mAlignSelf == NS_STYLE_ALIGN_SELF_AUTO) {
+    mAlignSelf =
+      mFrame->StyleContext()->GetParent()->StylePosition()->mAlignItems;
+  }
+
+  // If the flex item's inline axis is the same as the cross axis, then
+  // 'align-self:baseline' is identical to 'flex-start'. If that's the case, we
+  // just directly convert our align-self value here, so that we don't have to
+  // handle this with special cases elsewhere.
+  // Moreover: for the time being (until we support writing-modes),
+  // all inline axes are horizontal -- so we can just check if the cross axis
+  // is horizontal.
+  // FIXME: Once we support writing-mode (vertical text), this IsAxisHorizontal
+  // check won't be sufficient anymore -- we'll actually need to compare our
+  // inline axis vs. the cross axis.
+  if (mAlignSelf == NS_STYLE_ALIGN_ITEMS_BASELINE &&
+      IsAxisHorizontal(aAxisTracker.GetCrossAxis())) {
+    mAlignSelf = NS_STYLE_ALIGN_ITEMS_FLEX_START;
+  }
+}
+
+// Simplified constructor for creating a special "strut" FlexItem, for a child
+// with visibility:collapse. The strut has 0 main-size, and it only exists to
+// impose a minimum cross size on whichever FlexLine it ends up in.
+FlexItem::FlexItem(nsIFrame* aChildFrame, nscoord aCrossSize)
+  : mFrame(aChildFrame),
+    mFlexGrow(0.0f),
+    mFlexShrink(0.0f),
+    // mBorderPadding uses default constructor,
+    // mMargin uses default constructor,
+    mFlexBaseSize(0),
+    mMainMinSize(0),
+    mMainMaxSize(0),
+    mCrossMinSize(0),
+    mCrossMaxSize(0),
+    mMainSize(0),
+    mMainPosn(0),
+    mCrossSize(aCrossSize),
+    mCrossPosn(0),
+    mAscent(0),
+    mShareOfFlexWeightSoFar(0.0f),
+    mIsFrozen(true),
+    mHadMinViolation(false),
+    mHadMaxViolation(false),
+    mHadMeasuringReflow(false),
+    mIsStretched(false),
+    mIsStrut(true), // (this is the constructor for making struts, after all)
+    mAlignSelf(NS_STYLE_ALIGN_ITEMS_FLEX_START)
+{
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(NS_STYLE_VISIBILITY_COLLAPSE ==
+             mFrame->StyleVisibility()->mVisible,
+             "Should only make struts for children with 'visibility:collapse'");
+  MOZ_ASSERT(mFrame->GetType() != nsGkAtoms::placeholderFrame,
+             "placeholder frames should not be treated as flex items");
+  MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
+             "out-of-flow frames should not be treated as flex items");
+}
+
+nscoord
+FlexItem::GetBaselineOffsetFromOuterCrossEdge(AxisOrientationType aCrossAxis,
+                                              AxisEdgeType aEdge) const
+{
+  // NOTE: Currently, 'mAscent' (taken from reflow) is an inherently vertical
+  // measurement -- it's the distance from the border-top edge of this FlexItem
+  // to its baseline. So, we can really only do baseline alignment when the
+  // cross axis is vertical. (The FlexItem constructor enforces this when
+  // resolving the item's "mAlignSelf" value).
+  MOZ_ASSERT(!IsAxisHorizontal(aCrossAxis),
+             "Only expecting to be doing baseline computations when the "
+             "cross axis is vertical");
+
+  Side sideToMeasureFrom = kAxisOrientationToSidesMap[aCrossAxis][aEdge];
+
+  nscoord marginTopToBaseline = mAscent + mMargin.top;
+
+  if (sideToMeasureFrom == eSideTop) {
+    // Measuring from top (normal case): the distance from the margin-box top
+    // edge to the baseline is just ascent + margin-top.
+    return marginTopToBaseline;
+  }
+
+  MOZ_ASSERT(sideToMeasureFrom == eSideBottom,
+             "We already checked that we're dealing with a vertical axis, and "
+             "we're not using the top side, so that only leaves the bottom...");
+
+  // Measuring from bottom: The distance from the margin-box bottom edge to the
+  // baseline is just the margin-box cross size (i.e. outer cross size), minus
+  // the already-computed distance from margin-top to baseline.
+  return GetOuterCrossSize(aCrossAxis) - marginTopToBaseline;
+}
+
+uint32_t
+FlexItem::GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const
+{
+  uint32_t numAutoMargins = 0;
+  const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
+  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+    Side side = kAxisOrientationToSidesMap[aAxis][i];
+    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+      numAutoMargins++;
+    }
+  }
+
+  // Mostly for clarity:
+  MOZ_ASSERT(numAutoMargins <= 2,
+             "We're just looking at one item along one dimension, so we "
+             "should only have examined 2 margins");
+
+  return numAutoMargins;
+}
+
+// Keeps track of our position along a particular axis (where a '0' position
+// corresponds to the 'start' edge of that axis).
+// This class shouldn't be instantiated directly -- rather, it should only be
+// instantiated via its subclasses defined below.
+class MOZ_STACK_CLASS PositionTracker {
+public:
+  // Accessor for the current value of the position that we're tracking.
+  inline nscoord GetPosition() const { return mPosition; }
+  inline AxisOrientationType GetAxis() const { return mAxis; }
+
+  // Advances our position across the start edge of the given margin, in the
+  // axis we're tracking.
+  void EnterMargin(const nsMargin& aMargin)
+  {
+    Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_Start];
+    mPosition += MarginComponentForSide(aMargin, side);
+  }
+
+  // Advances our position across the end edge of the given margin, in the axis
+  // we're tracking.
+  void ExitMargin(const nsMargin& aMargin)
+  {
+    Side side = kAxisOrientationToSidesMap[mAxis][eAxisEdge_End];
+    mPosition += MarginComponentForSide(aMargin, side);
+  }
+
+  // Advances our current position from the start side of a child frame's
+  // border-box to the frame's upper or left edge (depending on our axis).
+  // (Note that this is a no-op if our axis grows in positive direction.)
+  void EnterChildFrame(nscoord aChildFrameSize)
+  {
+    if (!AxisGrowsInPositiveDirection(mAxis))
+      mPosition += aChildFrameSize;
+  }
+
+  // Advances our current position from a frame's upper or left border-box edge
+  // (whichever is in the axis we're tracking) to the 'end' side of the frame
+  // in the axis that we're tracking. (Note that this is a no-op if our axis
+  // grows in the negative direction.)
+  void ExitChildFrame(nscoord aChildFrameSize)
+  {
+    if (AxisGrowsInPositiveDirection(mAxis))
+      mPosition += aChildFrameSize;
+  }
+
+protected:
+  // Protected constructor, to be sure we're only instantiated via a subclass.
+  PositionTracker(AxisOrientationType aAxis)
+    : mPosition(0),
+      mAxis(aAxis)
+  {}
+
+private:
+  // Private copy-constructor, since we don't want any instances of our
+  // subclasses to be accidentally copied.
+  PositionTracker(const PositionTracker& aOther)
+    : mPosition(aOther.mPosition),
+      mAxis(aOther.mAxis)
+  {}
+
+protected:
+  // Member data:
+  nscoord mPosition;               // The position we're tracking
+  const AxisOrientationType mAxis; // The axis along which we're moving
+};
+
+// Tracks our position in the main axis, when we're laying out flex items.
+// The "0" position represents the main-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS MainAxisPositionTracker : public PositionTracker {
+public:
+  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
+                          const FlexLine* aLine,
+                          uint8_t aJustifyContent,
+                          nscoord aContentBoxMainSize);
+
+  ~MainAxisPositionTracker() {
+    MOZ_ASSERT(mNumPackingSpacesRemaining == 0,
+               "miscounted the number of packing spaces");
+    MOZ_ASSERT(mNumAutoMarginsInMainAxis == 0,
+               "miscounted the number of auto margins");
+  }
+
+  // Advances past the packing space (if any) between two flex items
+  void TraversePackingSpace();
+
+  // If aItem has any 'auto' margins in the main axis, this method updates the
+  // corresponding values in its margin.
+  void ResolveAutoMarginsInMainAxis(FlexItem& aItem);
+
+private:
+  nscoord  mPackingSpaceRemaining;
+  uint32_t mNumAutoMarginsInMainAxis;
+  uint32_t mNumPackingSpacesRemaining;
+  uint8_t  mJustifyContent;
+};
+
+// Utility class for managing our position along the cross axis along
+// the whole flex container (at a higher level than a single line).
+// The "0" position represents the cross-start edge of the flex container's
+// content-box.
+class MOZ_STACK_CLASS CrossAxisPositionTracker : public PositionTracker {
+public:
+  CrossAxisPositionTracker(FlexLine* aFirstLine,
+                           uint8_t aAlignContent,
+                           nscoord aContentBoxCrossSize,
+                           bool aIsCrossSizeDefinite,
+                           const FlexboxAxisTracker& aAxisTracker);
+
+  // Advances past the packing space (if any) between two flex lines
+  void TraversePackingSpace();
+
+  // Advances past the given FlexLine
+  void TraverseLine(FlexLine& aLine) { mPosition += aLine.GetLineCrossSize(); }
+
+private:
+  // Redeclare the frame-related methods from PositionTracker as private with
+  // MOZ_DELETE, to be sure (at compile time) that no client code can invoke
+  // them. (Unlike the other PositionTracker derived classes, this class here
+  // deals with FlexLines, not with individual FlexItems or frames.)
+  void EnterMargin(const nsMargin& aMargin) MOZ_DELETE;
+  void ExitMargin(const nsMargin& aMargin) MOZ_DELETE;
+  void EnterChildFrame(nscoord aChildFrameSize) MOZ_DELETE;
+  void ExitChildFrame(nscoord aChildFrameSize) MOZ_DELETE;
+
+  nscoord  mPackingSpaceRemaining;
+  uint32_t mNumPackingSpacesRemaining;
+  uint8_t  mAlignContent;
+};
+
+// Utility class for managing our position along the cross axis, *within* a
+// single flex line.
+class MOZ_STACK_CLASS SingleLineCrossAxisPositionTracker : public PositionTracker {
+public:
+  SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker);
+
+  void ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
+                                     FlexItem& aItem);
+
+  void EnterAlignPackingSpace(const FlexLine& aLine,
+                              const FlexItem& aItem,
+                              const FlexboxAxisTracker& aAxisTracker);
+
+  // Resets our position to the cross-start edge of this line.
+  inline void ResetPosition() { mPosition = 0; }
+};
+
+//----------------------------------------------------------------------
+
+// Frame class boilerplate
+// =======================
+
+NS_QUERYFRAME_HEAD(nsFlexContainerFrame)
+  NS_QUERYFRAME_ENTRY(nsFlexContainerFrame)
+NS_QUERYFRAME_TAIL_INHERITING(nsFlexContainerFrameSuper)
+
+NS_IMPL_FRAMEARENA_HELPERS(nsFlexContainerFrame)
+
+nsIFrame*
+NS_NewFlexContainerFrame(nsIPresShell* aPresShell,
+                         nsStyleContext* aContext)
+{
+  return new (aPresShell) nsFlexContainerFrame(aContext);
+}
+
+//----------------------------------------------------------------------
+
+// nsFlexContainerFrame Method Implementations
+// ===========================================
+
+/* virtual */
+nsFlexContainerFrame::~nsFlexContainerFrame()
+{
+}
+
+template<bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
+/* static */ bool
+nsFlexContainerFrame::SortChildrenIfNeeded()
+{
+  if (nsIFrame::IsFrameListSorted<IsLessThanOrEqual>(mFrames)) {
+    return false;
+  }
+
+  nsIFrame::SortFrameList<IsLessThanOrEqual>(mFrames);
+  return true;
+}
+
+/* virtual */
+nsIAtom*
+nsFlexContainerFrame::GetType() const
+{
+  return nsGkAtoms::flexContainerFrame;
+}
+
+#ifdef DEBUG_FRAME_DUMP
+nsresult
+nsFlexContainerFrame::GetFrameName(nsAString& aResult) const
+{
+  return MakeFrameName(NS_LITERAL_STRING("FlexContainer"), aResult);
+}
+#endif
+
+// Helper for BuildDisplayList, to implement this special-case for flex items
+// from the spec:
+//    Flex items paint exactly the same as block-level elements in the
+//    normal flow, except that 'z-index' values other than 'auto' create
+//    a stacking context even if 'position' is 'static'.
+// http://www.w3.org/TR/2012/CR-css3-flexbox-20120918/#painting
+uint32_t
+GetDisplayFlagsForFlexItem(nsIFrame* aFrame)
+{
+  MOZ_ASSERT(aFrame->IsFlexItem(), "Should only be called on flex items");
+
+  const nsStylePosition* pos = aFrame->StylePosition();
+  if (pos->mZIndex.GetUnit() == eStyleUnit_Integer) {
+    return nsIFrame::DISPLAY_CHILD_FORCE_STACKING_CONTEXT;
+  }
+  return nsIFrame::DISPLAY_CHILD_FORCE_PSEUDO_STACKING_CONTEXT;
+}
+
+void
+nsFlexContainerFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
+                                       const nsRect&           aDirtyRect,
+                                       const nsDisplayListSet& aLists)
+{
+  NS_ASSERTION(
+    nsIFrame::IsFrameListSorted<IsOrderLEQWithDOMFallback>(mFrames),
+    "Child frames aren't sorted correctly");
+
+  DisplayBorderBackgroundOutline(aBuilder, aLists);
+
+  // Our children are all block-level, so their borders/backgrounds all go on
+  // the BlockBorderBackgrounds list.
+  nsDisplayListSet childLists(aLists, aLists.BlockBorderBackgrounds());
+  for (nsFrameList::Enumerator e(mFrames); !e.AtEnd(); e.Next()) {
+    BuildDisplayListForChild(aBuilder, e.get(), aDirtyRect, childLists,
+                             GetDisplayFlagsForFlexItem(e.get()));
+  }
+}
+
+#ifdef DEBUG
+// helper for the debugging method below
+bool
+FrameWantsToBeInAnonymousFlexItem(nsIFrame* aFrame)
+{
+  // Note: This needs to match the logic in
+  // nsCSSFrameConstructor::FrameConstructionItem::NeedsAnonFlexItem()
+  return (aFrame->IsFrameOfType(nsIFrame::eLineParticipant) ||
+          nsGkAtoms::placeholderFrame == aFrame->GetType());
+}
+
+// Debugging method, to let us assert that our anonymous flex items are
+// set up correctly -- in particular, we assert:
+//  (1) we don't have any inline non-replaced children
+//  (2) we don't have any consecutive anonymous flex items
+//  (3) we don't have any empty anonymous flex items
+//
+// XXXdholbert This matches what nsCSSFrameConstructor currently does, and what
+// the spec used to say.  However, the spec has now changed regarding what
+// types of content get wrapped in an anonymous flexbox item.  The patch that
+// implements those changes (in nsCSSFrameConstructor) will need to change
+// this method as well.
+void
+nsFlexContainerFrame::SanityCheckAnonymousFlexItems() const
+{
+  bool prevChildWasAnonFlexItem = false;
+  for (nsIFrame* child = mFrames.FirstChild(); child;
+       child = child->GetNextSibling()) {
+    MOZ_ASSERT(!FrameWantsToBeInAnonymousFlexItem(child),
+               "frame wants to be inside an anonymous flex item, "
+               "but it isn't");
+    if (child->StyleContext()->GetPseudo() ==
+        nsCSSAnonBoxes::anonymousFlexItem) {
+      MOZ_ASSERT(!prevChildWasAnonFlexItem ||
+                 HasAnyStateBits(NS_STATE_FLEX_CHILDREN_REORDERED),
+                 "two anon flex items in a row (shouldn't happen, unless our "
+                 "children have been reordered with the 'order' property)");
+
+      nsIFrame* firstWrappedChild = child->GetFirstPrincipalChild();
+      MOZ_ASSERT(firstWrappedChild,
+                 "anonymous flex item is empty (shouldn't happen)");
+      prevChildWasAnonFlexItem = true;
+    } else {
+      prevChildWasAnonFlexItem = false;
+    }
+  }
+}
+#endif // DEBUG
+
+// Based on the sign of aTotalViolation, this function freezes a subset of our
+// flexible sizes, and restores the remaining ones to their initial pref sizes.
+void
+FlexLine::FreezeOrRestoreEachFlexibleSize(const nscoord aTotalViolation,
+                                          bool aIsFinalIteration)
+{
+  enum FreezeType {
+    eFreezeEverything,
+    eFreezeMinViolations,
+    eFreezeMaxViolations
+  };
+
+  FreezeType freezeType;
+  if (aTotalViolation == 0) {
+    freezeType = eFreezeEverything;
+  } else if (aTotalViolation > 0) {
+    freezeType = eFreezeMinViolations;
+  } else { // aTotalViolation < 0
+    freezeType = eFreezeMaxViolations;
+  }
+
+  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    MOZ_ASSERT(!item->HadMinViolation() || !item->HadMaxViolation(),
+               "Can have either min or max violation, but not both");
+
+    if (!item->IsFrozen()) {
+      if (eFreezeEverything == freezeType ||
+          (eFreezeMinViolations == freezeType && item->HadMinViolation()) ||
+          (eFreezeMaxViolations == freezeType && item->HadMaxViolation())) {
+
+        MOZ_ASSERT(item->GetMainSize() >= item->GetMainMinSize(),
+                   "Freezing item at a size below its minimum");
+        MOZ_ASSERT(item->GetMainSize() <= item->GetMainMaxSize(),
+                   "Freezing item at a size above its maximum");
+
+        item->Freeze();
+      } else if (MOZ_UNLIKELY(aIsFinalIteration)) {
+        // XXXdholbert If & when bug 765861 is fixed, we should upgrade this
+        // assertion to be fatal except in documents with enormous lengths.
+        NS_ERROR("Final iteration still has unfrozen items, this shouldn't"
+                 " happen unless there was nscoord under/overflow.");
+        item->Freeze();
+      } // else, we'll reset this item's main size to its flex base size on the
+        // next iteration of this algorithm.
+
+      // Clear this item's violation(s), now that we've dealt with them
+      item->ClearViolationFlags();
+    }
+  }
+}
+
+void
+FlexLine::ResolveFlexibleLengths(nscoord aFlexContainerMainSize)
+{
+  PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG, ("ResolveFlexibleLengths\n"));
+  if (IsEmpty()) {
+    return;
+  }
+
+  // Subtract space occupied by our items' margins/borders/padding, so we can
+  // just be dealing with the space available for our flex items' content
+  // boxes.
+  nscoord spaceReservedForMarginBorderPadding =
+    mTotalOuterHypotheticalMainSize - mTotalInnerHypotheticalMainSize;
+
+  nscoord spaceAvailableForFlexItemsContentBoxes =
+    aFlexContainerMainSize - spaceReservedForMarginBorderPadding;
+
+  // Determine whether we're going to be growing or shrinking items.
+  const bool isUsingFlexGrow =
+    (mTotalOuterHypotheticalMainSize < aFlexContainerMainSize);
+
+  // NOTE: I claim that this chunk of the algorithm (the looping part) needs to
+  // run the loop at MOST mNumItems times.  This claim should hold up
+  // because we'll freeze at least one item on each loop iteration, and once
+  // we've run out of items to freeze, there's nothing left to do.  However,
+  // in most cases, we'll break out of this loop long before we hit that many
+  // iterations.
+  for (uint32_t iterationCounter = 0;
+       iterationCounter < mNumItems; iterationCounter++) {
+    // Set every not-yet-frozen item's used main size to its
+    // flex base size, and subtract all the used main sizes from our
+    // total amount of space to determine the 'available free space'
+    // (positive or negative) to be distributed among our flexible items.
+    nscoord availableFreeSpace = spaceAvailableForFlexItemsContentBoxes;
+    for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+      if (!item->IsFrozen()) {
+        item->SetMainSize(item->GetFlexBaseSize());
+      }
+      availableFreeSpace -= item->GetMainSize();
+    }
+
+    PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+           (" available free space = %d\n", availableFreeSpace));
+
+    // If sign of free space matches the type of flexing that we're doing, give
+    // each flexible item a portion of availableFreeSpace.
+    if ((availableFreeSpace > 0 && isUsingFlexGrow) ||
+        (availableFreeSpace < 0 && !isUsingFlexGrow)) {
+
+      // STRATEGY: On each item, we compute & store its "share" of the total
+      // flex weight that we've seen so far:
+      //   curFlexWeight / runningFlexWeightSum
+      //
+      // Then, when we go to actually distribute the space (in the next loop),
+      // we can simply walk backwards through the elements and give each item
+      // its "share" multiplied by the remaining available space.
+      //
+      // SPECIAL CASE: If the sum of the flex weights is larger than the
+      // maximum representable float (overflowing to infinity), then we can't
+      // sensibly divide out proportional shares anymore. In that case, we
+      // simply treat the flex item(s) with the largest flex weights as if
+      // their weights were infinite (dwarfing all the others), and we
+      // distribute all of the available space among them.
+      float runningFlexWeightSum = 0.0f;
+      float largestFlexWeight = 0.0f;
+      uint32_t numItemsWithLargestFlexWeight = 0;
+      for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+        float curFlexWeight = item->GetFlexWeightToUse(isUsingFlexGrow);
+        MOZ_ASSERT(curFlexWeight >= 0.0f, "weights are non-negative");
+
+        runningFlexWeightSum += curFlexWeight;
+        if (NS_finite(runningFlexWeightSum)) {
+          if (curFlexWeight == 0.0f) {
+            item->SetShareOfFlexWeightSoFar(0.0f);
+          } else {
+            item->SetShareOfFlexWeightSoFar(curFlexWeight /
+                                            runningFlexWeightSum);
+          }
+        } // else, the sum of weights overflows to infinity, in which
+          // case we don't bother with "SetShareOfFlexWeightSoFar" since
+          // we know we won't use it. (instead, we'll just give every
+          // item with the largest flex weight an equal share of space.)
+
+        // Update our largest-flex-weight tracking vars
+        if (curFlexWeight > largestFlexWeight) {
+          largestFlexWeight = curFlexWeight;
+          numItemsWithLargestFlexWeight = 1;
+        } else if (curFlexWeight == largestFlexWeight) {
+          numItemsWithLargestFlexWeight++;
+        }
+      }
+
+      if (runningFlexWeightSum != 0.0f) { // no distribution if no flexibility
+        PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+               (" Distributing available space:"));
+        // NOTE: It's important that we traverse our items in *reverse* order
+        // here, for correct width distribution according to the items'
+        // "ShareOfFlexWeightSoFar" progressively-calculated values.
+        for (FlexItem* item = mItems.getLast(); item;
+             item = item->getPrevious()) {
+
+          if (!item->IsFrozen()) {
+            // To avoid rounding issues, we compute the change in size for this
+            // item, and then subtract it from the remaining available space.
+            nscoord sizeDelta = 0;
+            if (NS_finite(runningFlexWeightSum)) {
+              float myShareOfRemainingSpace =
+                item->GetShareOfFlexWeightSoFar();
+
+              MOZ_ASSERT(myShareOfRemainingSpace >= 0.0f &&
+                         myShareOfRemainingSpace <= 1.0f,
+                         "my share should be nonnegative fractional amount");
+
+              if (myShareOfRemainingSpace == 1.0f) {
+                // (We special-case 1.0f to avoid float error from converting
+                // availableFreeSpace from integer*1.0f --> float --> integer)
+                sizeDelta = availableFreeSpace;
+              } else if (myShareOfRemainingSpace > 0.0f) {
+                sizeDelta = NSToCoordRound(availableFreeSpace *
+                                           myShareOfRemainingSpace);
+              }
+            } else if (item->GetFlexWeightToUse(isUsingFlexGrow) ==
+                       largestFlexWeight) {
+              // Total flexibility is infinite, so we're just distributing
+              // the available space equally among the items that are tied for
+              // having the largest weight (and this is one of those items).
+              sizeDelta =
+                NSToCoordRound(availableFreeSpace /
+                               float(numItemsWithLargestFlexWeight));
+              numItemsWithLargestFlexWeight--;
+            }
+
+            availableFreeSpace -= sizeDelta;
+
+            item->SetMainSize(item->GetMainSize() + sizeDelta);
+            PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+                   ("  child %p receives %d, for a total of %d\n",
+                    item, sizeDelta, item->GetMainSize()));
+          }
+        }
+      }
+    }
+
+    // Fix min/max violations:
+    nscoord totalViolation = 0; // keeps track of adjustments for min/max
+    PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+           (" Checking for violations:"));
+
+    for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+      if (!item->IsFrozen()) {
+        if (item->GetMainSize() < item->GetMainMinSize()) {
+          // min violation
+          totalViolation += item->GetMainMinSize() - item->GetMainSize();
+          item->SetMainSize(item->GetMainMinSize());
+          item->SetHadMinViolation();
+        } else if (item->GetMainSize() > item->GetMainMaxSize()) {
+          // max violation
+          totalViolation += item->GetMainMaxSize() - item->GetMainSize();
+          item->SetMainSize(item->GetMainMaxSize());
+          item->SetHadMaxViolation();
+        }
+      }
+    }
+
+    FreezeOrRestoreEachFlexibleSize(totalViolation,
+                                    iterationCounter + 1 == mNumItems);
+
+    PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+           (" Total violation: %d\n", totalViolation));
+
+    if (totalViolation == 0) {
+      break;
+    }
+  }
+
+  // Post-condition: all lengths should've been frozen.
+#ifdef DEBUG
+  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    MOZ_ASSERT(item->IsFrozen(),
+               "All flexible lengths should've been resolved");
+  }
+#endif // DEBUG
+}
+
+MainAxisPositionTracker::
+  MainAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker,
+                          const FlexLine* aLine,
+                          uint8_t aJustifyContent,
+                          nscoord aContentBoxMainSize)
+  : PositionTracker(aAxisTracker.GetMainAxis()),
+    mPackingSpaceRemaining(aContentBoxMainSize), // we chip away at this below
+    mNumAutoMarginsInMainAxis(0),
+    mNumPackingSpacesRemaining(0),
+    mJustifyContent(aJustifyContent)
+{
+  // mPackingSpaceRemaining is initialized to the container's main size.  Now
+  // we'll subtract out the main sizes of our flex items, so that it ends up
+  // with the *actual* amount of packing space.
+  for (const FlexItem* item = aLine->GetFirstItem(); item;
+       item = item->getNext()) {
+    mPackingSpaceRemaining -= item->GetOuterMainSize(mAxis);
+    mNumAutoMarginsInMainAxis += item->GetNumAutoMarginsInAxis(mAxis);
+  }
+
+  if (mPackingSpaceRemaining <= 0) {
+    // No available packing space to use for resolving auto margins.
+    mNumAutoMarginsInMainAxis = 0;
+  }
+
+  // If packing space is negative, 'space-between' behaves like 'flex-start',
+  // and 'space-around' behaves like 'center'. In those cases, it's simplest to
+  // just pretend we have a different 'justify-content' value and share code.
+  if (mPackingSpaceRemaining < 0) {
+    if (mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_SPACE_BETWEEN) {
+      mJustifyContent = NS_STYLE_JUSTIFY_CONTENT_FLEX_START;
+    } else if (mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_SPACE_AROUND) {
+      mJustifyContent = NS_STYLE_JUSTIFY_CONTENT_CENTER;
+    }
+  }
+
+  // If our main axis is (internally) reversed, swap the justify-content
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_FLEX_START) {
+      mJustifyContent = NS_STYLE_JUSTIFY_CONTENT_FLEX_END;
+    } else if (mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_FLEX_END) {
+      mJustifyContent = NS_STYLE_JUSTIFY_CONTENT_FLEX_START;
+    }
+  }
+
+  // Figure out how much space we'll set aside for auto margins or
+  // packing spaces, and advance past any leading packing-space.
+  if (mNumAutoMarginsInMainAxis == 0 &&
+      mPackingSpaceRemaining != 0 &&
+      !aLine->IsEmpty()) {
+    switch (mJustifyContent) {
+      case NS_STYLE_JUSTIFY_CONTENT_FLEX_START:
+        // All packing space should go at the end --> nothing to do here.
+        break;
+      case NS_STYLE_JUSTIFY_CONTENT_FLEX_END:
+        // All packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining;
+        break;
+      case NS_STYLE_JUSTIFY_CONTENT_CENTER:
+        // Half the packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining / 2;
+        break;
+      case NS_STYLE_JUSTIFY_CONTENT_SPACE_BETWEEN:
+        MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+                   "negative packing space should make us use 'flex-start' "
+                   "instead of 'space-between'");
+        // 1 packing space between each flex item, no packing space at ends.
+        mNumPackingSpacesRemaining = aLine->NumItems() - 1;
+        break;
+      case NS_STYLE_JUSTIFY_CONTENT_SPACE_AROUND:
+        MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+                   "negative packing space should make us use 'center' "
+                   "instead of 'space-around'");
+        // 1 packing space between each flex item, plus half a packing space
+        // at beginning & end.  So our number of full packing-spaces is equal
+        // to the number of flex items.
+        mNumPackingSpacesRemaining = aLine->NumItems();
+        if (mNumPackingSpacesRemaining > 0) {
+          // The edges (start/end) share one full packing space
+          nscoord totalEdgePackingSpace =
+            mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+          // ...and we'll use half of that right now, at the start
+          mPosition += totalEdgePackingSpace / 2;
+          // ...but we need to subtract all of it right away, so that we won't
+          // hand out any of it to intermediate packing spaces.
+          mPackingSpaceRemaining -= totalEdgePackingSpace;
+          mNumPackingSpacesRemaining--;
+        }
+        break;
+      default:
+        MOZ_CRASH("Unexpected justify-content value");
+    }
+  }
+
+  MOZ_ASSERT(mNumPackingSpacesRemaining == 0 ||
+             mNumAutoMarginsInMainAxis == 0,
+             "extra space should either go to packing space or to "
+             "auto margins, but not to both");
+}
+
+void
+MainAxisPositionTracker::ResolveAutoMarginsInMainAxis(FlexItem& aItem)
+{
+  if (mNumAutoMarginsInMainAxis) {
+    const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+    for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+      Side side = kAxisOrientationToSidesMap[mAxis][i];
+      if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+        // NOTE: This integer math will skew the distribution of remainder
+        // app-units towards the end, which is fine.
+        nscoord curAutoMarginSize =
+          mPackingSpaceRemaining / mNumAutoMarginsInMainAxis;
+
+        MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                   "Expecting auto margins to have value '0' before we "
+                   "resolve them");
+        aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+
+        mNumAutoMarginsInMainAxis--;
+        mPackingSpaceRemaining -= curAutoMarginSize;
+      }
+    }
+  }
+}
+
+void
+MainAxisPositionTracker::TraversePackingSpace()
+{
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_SPACE_BETWEEN ||
+               mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_SPACE_AROUND,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+      mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+CrossAxisPositionTracker::
+  CrossAxisPositionTracker(FlexLine* aFirstLine,
+                           uint8_t aAlignContent,
+                           nscoord aContentBoxCrossSize,
+                           bool aIsCrossSizeDefinite,
+                           const FlexboxAxisTracker& aAxisTracker)
+  : PositionTracker(aAxisTracker.GetCrossAxis()),
+    mPackingSpaceRemaining(0),
+    mNumPackingSpacesRemaining(0),
+    mAlignContent(aAlignContent)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+
+  if (aIsCrossSizeDefinite && !aFirstLine->getNext()) {
+    // "If the flex container has only a single line (even if it's a
+    // multi-line flex container) and has a definite cross size, the cross
+    // size of the flex line is the flex container's inner cross size."
+    // SOURCE: http://dev.w3.org/csswg/css-flexbox/#algo-line-break
+    // NOTE: This means (by definition) that there's no packing space, which
+    // means we don't need to be concerned with "align-conent" at all and we
+    // can return early. This is handy, because this is the usual case (for
+    // single-line flexbox).
+    aFirstLine->SetLineCrossSize(aContentBoxCrossSize);
+    return;
+  }
+
+  // NOTE: The rest of this function should essentially match
+  // MainAxisPositionTracker's constructor, though with FlexLines instead of
+  // FlexItems, and with the additional value "stretch" (and of course with
+  // cross sizes instead of main sizes.)
+
+  // Figure out how much packing space we have (container's cross size minus
+  // all the lines' cross sizes).  Also, share this loop to count how many
+  // lines we have. (We need that count in some cases below.)
+  mPackingSpaceRemaining = aContentBoxCrossSize;
+  uint32_t numLines = 0;
+  for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    mPackingSpaceRemaining -= line->GetLineCrossSize();
+    numLines++;
+  }
+
+  // If packing space is negative, 'space-between' and 'stretch' behave like
+  // 'flex-start', and 'space-around' behaves like 'center'. In those cases,
+  // it's simplest to just pretend we have a different 'align-content' value
+  // and share code.
+  if (mPackingSpaceRemaining < 0) {
+    if (mAlignContent == NS_STYLE_ALIGN_CONTENT_SPACE_BETWEEN ||
+        mAlignContent == NS_STYLE_ALIGN_CONTENT_STRETCH) {
+      mAlignContent = NS_STYLE_ALIGN_CONTENT_FLEX_START;
+    } else if (mAlignContent == NS_STYLE_ALIGN_CONTENT_SPACE_AROUND) {
+      mAlignContent = NS_STYLE_ALIGN_CONTENT_CENTER;
+    }
+  }
+
+  // If our cross axis is (internally) reversed, swap the align-content
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (mAlignContent == NS_STYLE_ALIGN_CONTENT_FLEX_START) {
+      mAlignContent = NS_STYLE_ALIGN_CONTENT_FLEX_END;
+    } else if (mAlignContent == NS_STYLE_ALIGN_CONTENT_FLEX_END) {
+      mAlignContent = NS_STYLE_ALIGN_CONTENT_FLEX_START;
+    }
+  }
+
+  // Figure out how much space we'll set aside for packing spaces, and advance
+  // past any leading packing-space.
+  if (mPackingSpaceRemaining != 0) {
+    switch (mAlignContent) {
+      case NS_STYLE_ALIGN_CONTENT_FLEX_START:
+        // All packing space should go at the end --> nothing to do here.
+        break;
+      case NS_STYLE_ALIGN_CONTENT_FLEX_END:
+        // All packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining;
+        break;
+      case NS_STYLE_ALIGN_CONTENT_CENTER:
+        // Half the packing space goes at the beginning
+        mPosition += mPackingSpaceRemaining / 2;
+        break;
+      case NS_STYLE_ALIGN_CONTENT_SPACE_BETWEEN:
+        MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+                   "negative packing space should make us use 'flex-start' "
+                   "instead of 'space-between'");
+        // 1 packing space between each flex line, no packing space at ends.
+        mNumPackingSpacesRemaining = numLines - 1;
+        break;
+      case NS_STYLE_ALIGN_CONTENT_SPACE_AROUND: {
+        MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+                   "negative packing space should make us use 'center' "
+                   "instead of 'space-around'");
+        // 1 packing space between each flex line, plus half a packing space
+        // at beginning & end.  So our number of full packing-spaces is equal
+        // to the number of flex lines.
+        mNumPackingSpacesRemaining = numLines;
+        // The edges (start/end) share one full packing space
+        nscoord totalEdgePackingSpace =
+          mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+        // ...and we'll use half of that right now, at the start
+        mPosition += totalEdgePackingSpace / 2;
+        // ...but we need to subtract all of it right away, so that we won't
+        // hand out any of it to intermediate packing spaces.
+        mPackingSpaceRemaining -= totalEdgePackingSpace;
+        mNumPackingSpacesRemaining--;
+        break;
+      }
+      case NS_STYLE_ALIGN_CONTENT_STRETCH: {
+        // Split space equally between the lines:
+        MOZ_ASSERT(mPackingSpaceRemaining > 0,
+                   "negative packing space should make us use 'flex-start' "
+                   "instead of 'stretch' (and we shouldn't bother with this "
+                   "code if we have 0 packing space)");
+
+        uint32_t numLinesLeft = numLines;
+        for (FlexLine* line = aFirstLine; line; line = line->getNext()) {
+          // Our share is the amount of space remaining, divided by the number
+          // of lines remainig.
+          MOZ_ASSERT(numLinesLeft > 0, "miscalculated num lines");
+          nscoord shareOfExtraSpace = mPackingSpaceRemaining / numLinesLeft;
+          nscoord newSize = line->GetLineCrossSize() + shareOfExtraSpace;
+          line->SetLineCrossSize(newSize);
+
+          mPackingSpaceRemaining -= shareOfExtraSpace;
+          numLinesLeft--;
+        }
+        MOZ_ASSERT(numLinesLeft == 0, "miscalculated num lines");
+        break;
+      }
+      default:
+        MOZ_CRASH("Unexpected align-content value");
+    }
+  }
+}
+
+void
+CrossAxisPositionTracker::TraversePackingSpace()
+{
+  if (mNumPackingSpacesRemaining) {
+    MOZ_ASSERT(mAlignContent == NS_STYLE_ALIGN_CONTENT_SPACE_BETWEEN ||
+               mAlignContent == NS_STYLE_ALIGN_CONTENT_SPACE_AROUND,
+               "mNumPackingSpacesRemaining only applies for "
+               "space-between/space-around");
+
+    MOZ_ASSERT(mPackingSpaceRemaining >= 0,
+               "ran out of packing space earlier than we expected");
+
+    // NOTE: This integer math will skew the distribution of remainder
+    // app-units towards the end, which is fine.
+    nscoord curPackingSpace =
+      mPackingSpaceRemaining / mNumPackingSpacesRemaining;
+
+    mPosition += curPackingSpace;
+    mNumPackingSpacesRemaining--;
+    mPackingSpaceRemaining -= curPackingSpace;
+  }
+}
+
+SingleLineCrossAxisPositionTracker::
+  SingleLineCrossAxisPositionTracker(const FlexboxAxisTracker& aAxisTracker)
+  : PositionTracker(aAxisTracker.GetCrossAxis())
+{
+}
+
+void
+FlexLine::ComputeCrossSizeAndBaseline(const FlexboxAxisTracker& aAxisTracker)
+{
+  nscoord crossStartToFurthestBaseline = nscoord_MIN;
+  nscoord crossEndToFurthestBaseline = nscoord_MIN;
+  nscoord largestOuterCrossSize = 0;
+  for (const FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    nscoord curOuterCrossSize =
+      item->GetOuterCrossSize(aAxisTracker.GetCrossAxis());
+
+    if (item->GetAlignSelf() == NS_STYLE_ALIGN_ITEMS_BASELINE &&
+        item->GetNumAutoMarginsInAxis(aAxisTracker.GetCrossAxis()) == 0) {
+      // FIXME: Once we support "writing-mode", we'll have to do baseline
+      // alignment in vertical flex containers here (w/ horizontal cross-axes).
+
+      // Find distance from our item's cross-start and cross-end margin-box
+      // edges to its baseline.
+      //
+      // Here's a diagram of a flex-item that we might be doing this on.
+      // "mmm" is the margin-box, "bbb" is the border-box. The bottom of
+      // the text "BASE" is the baseline.
+      //
+      // ---(cross-start)---
+      //                ___              ___            ___
+      //   mmmmmmmmmmmm  |                |margin-start  |
+      //   m          m  |               _|_   ___       |
+      //   m bbbbbbbb m  |curOuterCrossSize     |        |crossStartToBaseline
+      //   m b      b m  |                      |ascent  |
+      //   m b BASE b m  |                     _|_      _|_
+      //   m b      b m  |                               |
+      //   m bbbbbbbb m  |                               |crossEndToBaseline
+      //   m          m  |                               |
+      //   mmmmmmmmmmmm _|_                             _|_
+      //
+      // ---(cross-end)---
+      //
+      // We already have the curOuterCrossSize, margin-start, and the ascent.
+      // * We can get crossStartToBaseline by adding margin-start + ascent.
+      // * If we subtract that from the curOuterCrossSize, we get
+      //   crossEndToBaseline.
+
+      nscoord crossStartToBaseline =
+        item->GetBaselineOffsetFromOuterCrossEdge(aAxisTracker.GetCrossAxis(),
+                                                  eAxisEdge_Start);
+      nscoord crossEndToBaseline = curOuterCrossSize - crossStartToBaseline;
+
+      // Now, update our "largest" values for these (across all the flex items
+      // in this flex line), so we can use them in computing the line's cross
+      // size below:
+      crossStartToFurthestBaseline = std::max(crossStartToFurthestBaseline,
+                                              crossStartToBaseline);
+      crossEndToFurthestBaseline = std::max(crossEndToFurthestBaseline,
+                                            crossEndToBaseline);
+    } else {
+      largestOuterCrossSize = std::max(largestOuterCrossSize, curOuterCrossSize);
+    }
+  }
+
+  // The line's baseline offset is the distance from the line's edge (start or
+  // end, depending on whether we've flipped the axes) to the furthest
+  // item-baseline. The item(s) with that baseline will be exactly aligned with
+  // the line's edge.
+  mBaselineOffset = aAxisTracker.AreAxesInternallyReversed() ?
+    crossEndToFurthestBaseline : crossStartToFurthestBaseline;
+
+  // The line's cross-size is the larger of:
+  //  (a) [largest cross-start-to-baseline + largest baseline-to-cross-end] of
+  //      all baseline-aligned items with no cross-axis auto margins...
+  // and
+  //  (b) largest cross-size of all other children.
+  mLineCrossSize = std::max(crossStartToFurthestBaseline +
+                            crossEndToFurthestBaseline,
+                            largestOuterCrossSize);
+}
+
+void
+FlexItem::ResolveStretchedCrossSize(nscoord aLineCrossSize,
+                                    const FlexboxAxisTracker& aAxisTracker)
+{
+  AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
+  // We stretch IFF we are align-self:stretch, have no auto margins in
+  // cross axis, and have cross-axis size property == "auto". If any of those
+  // conditions don't hold up, we won't stretch.
+  if (mAlignSelf != NS_STYLE_ALIGN_ITEMS_STRETCH ||
+      GetNumAutoMarginsInAxis(crossAxis) != 0 ||
+      eStyleUnit_Auto != GetSizePropertyForAxis(mFrame, crossAxis).GetUnit()) {
+    return;
+  }
+
+  // If we've already been stretched, we can bail out early, too.
+  // No need to redo the calculation.
+  if (mIsStretched) {
+    return;
+  }
+
+  // Reserve space for margins & border & padding, and then use whatever
+  // remains as our item's cross-size (clamped to its min/max range).
+  nscoord stretchedSize = aLineCrossSize -
+    GetMarginBorderPaddingSizeInAxis(crossAxis);
+
+  stretchedSize = NS_CSS_MINMAX(stretchedSize, mCrossMinSize, mCrossMaxSize);
+
+  // Update the cross-size & make a note that it's stretched, so we know to
+  // override the reflow state's computed cross-size in our final reflow.
+  SetCrossSize(stretchedSize);
+  mIsStretched = true;
+}
+
+void
+SingleLineCrossAxisPositionTracker::
+  ResolveAutoMarginsInCrossAxis(const FlexLine& aLine,
+                                FlexItem& aItem)
+{
+  // Subtract the space that our item is already occupying, to see how much
+  // space (if any) is available for its auto margins.
+  nscoord spaceForAutoMargins = aLine.GetLineCrossSize() -
+    aItem.GetOuterCrossSize(mAxis);
+
+  if (spaceForAutoMargins <= 0) {
+    return; // No available space  --> nothing to do
+  }
+
+  uint32_t numAutoMargins = aItem.GetNumAutoMarginsInAxis(mAxis);
+  if (numAutoMargins == 0) {
+    return; // No auto margins --> nothing to do.
+  }
+
+  // OK, we have at least one auto margin and we have some available space.
+  // Give each auto margin a share of the space.
+  const nsStyleSides& styleMargin = aItem.Frame()->StyleMargin()->mMargin;
+  for (uint32_t i = 0; i < eNumAxisEdges; i++) {
+    Side side = kAxisOrientationToSidesMap[mAxis][i];
+    if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
+      MOZ_ASSERT(aItem.GetMarginComponentForSide(side) == 0,
+                 "Expecting auto margins to have value '0' before we "
+                 "update them");
+
+      // NOTE: integer divison is fine here; numAutoMargins is either 1 or 2.
+      // If it's 2 & spaceForAutoMargins is odd, 1st margin gets smaller half.
+      nscoord curAutoMarginSize = spaceForAutoMargins / numAutoMargins;
+      aItem.SetMarginComponentForSide(side, curAutoMarginSize);
+      numAutoMargins--;
+      spaceForAutoMargins -= curAutoMarginSize;
+    }
+  }
+}
+
+void
+SingleLineCrossAxisPositionTracker::
+  EnterAlignPackingSpace(const FlexLine& aLine,
+                         const FlexItem& aItem,
+                         const FlexboxAxisTracker& aAxisTracker)
+{
+  // We don't do align-self alignment on items that have auto margins
+  // in the cross axis.
+  if (aItem.GetNumAutoMarginsInAxis(mAxis)) {
+    return;
+  }
+
+  uint8_t alignSelf = aItem.GetAlignSelf();
+  // NOTE: 'stretch' behaves like 'flex-start' once we've stretched any
+  // auto-sized items (which we've already done).
+  if (alignSelf == NS_STYLE_ALIGN_ITEMS_STRETCH) {
+    alignSelf = NS_STYLE_ALIGN_ITEMS_FLEX_START;
+  }
+
+  // If our cross axis is (internally) reversed, swap the align-self
+  // "flex-start" and "flex-end" behaviors:
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    if (alignSelf == NS_STYLE_ALIGN_ITEMS_FLEX_START) {
+      alignSelf = NS_STYLE_ALIGN_ITEMS_FLEX_END;
+    } else if (alignSelf == NS_STYLE_ALIGN_ITEMS_FLEX_END) {
+      alignSelf = NS_STYLE_ALIGN_ITEMS_FLEX_START;
+    }
+  }
+
+  switch (alignSelf) {
+    case NS_STYLE_ALIGN_ITEMS_FLEX_START:
+      // No space to skip over -- we're done.
+      break;
+    case NS_STYLE_ALIGN_ITEMS_FLEX_END:
+      mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
+      break;
+    case NS_STYLE_ALIGN_ITEMS_CENTER:
+      // Note: If cross-size is odd, the "after" space will get the extra unit.
+      mPosition +=
+        (aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis)) / 2;
+      break;
+    case NS_STYLE_ALIGN_ITEMS_BASELINE: {
+      // Normally, baseline-aligned items are collectively aligned with the
+      // line's cross-start edge; however, if our cross axis is (internally)
+      // reversed, we instead align them with the cross-end edge.
+      nscoord itemBaselineOffset =
+        aItem.GetBaselineOffsetFromOuterCrossEdge(mAxis,
+          aAxisTracker.AreAxesInternallyReversed() ?
+          eAxisEdge_End : eAxisEdge_Start);
+
+      nscoord lineBaselineOffset = aLine.GetBaselineOffset();
+
+      NS_ASSERTION(lineBaselineOffset >= itemBaselineOffset,
+                   "failed at finding largest baseline offset");
+
+      // How much do we need to adjust our position (from the line edge),
+      // to get the item's baseline to hit the line's baseline offset:
+      nscoord baselineDiff = lineBaselineOffset - itemBaselineOffset;
+
+      if (aAxisTracker.AreAxesInternallyReversed()) {
+        // Advance to align item w/ line's flex-end edge (as in FLEX_END case):
+        mPosition += aLine.GetLineCrossSize() - aItem.GetOuterCrossSize(mAxis);
+        // ...and step *back* by the baseline adjustment:
+        mPosition -= baselineDiff;
+      } else {
+        // mPosition is already at line's flex-start edge.
+        // From there, we step *forward* by the baseline adjustment:
+        mPosition += baselineDiff;
+      }
+      break;
+    }
+    default:
+      NS_NOTREACHED("Unexpected align-self value");
+      break;
+  }
+}
+
+FlexboxAxisTracker::FlexboxAxisTracker(
+  nsFlexContainerFrame* aFlexContainerFrame)
+  : mAreAxesInternallyReversed(false)
+{
+  const nsStylePosition* pos = aFlexContainerFrame->StylePosition();
+  uint32_t flexDirection = pos->mFlexDirection;
+  uint32_t cssDirection =
+    aFlexContainerFrame->StyleVisibility()->mDirection;
+
+  MOZ_ASSERT(cssDirection == NS_STYLE_DIRECTION_LTR ||
+             cssDirection == NS_STYLE_DIRECTION_RTL,
+             "Unexpected computed value for 'direction' property");
+  // (Not asserting for flexDirection here; it's checked by the switch below.)
+
+  // These are defined according to writing-modes' definitions of
+  // start/end (for the inline dimension) and before/after (for the block
+  // dimension), here:
+  //   http://www.w3.org/TR/css3-writing-modes/#logical-directions
+  // (NOTE: I'm intentionally not calling this "inlineAxis"/"blockAxis", since
+  // those terms have explicit definition in the writing-modes spec, which are
+  // the opposite of how I'd be using them here.)
+  // XXXdholbert Once we support the 'writing-mode' property, use its value
+  // here to further customize inlineDimension & blockDimension.
+
+  // Inline dimension ("start-to-end"):
+  AxisOrientationType inlineDimension =
+    cssDirection == NS_STYLE_DIRECTION_RTL ? eAxis_RL : eAxis_LR;
+
+  // Block dimension ("before-to-after"):
+  AxisOrientationType blockDimension = eAxis_TB;
+
+  // Determine main axis:
+  switch (flexDirection) {
+    case NS_STYLE_FLEX_DIRECTION_ROW:
+      mMainAxis = inlineDimension;
+      break;
+    case NS_STYLE_FLEX_DIRECTION_ROW_REVERSE:
+      mMainAxis = GetReverseAxis(inlineDimension);
+      break;
+    case NS_STYLE_FLEX_DIRECTION_COLUMN:
+      mMainAxis = blockDimension;
+      break;
+    case NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE:
+      mMainAxis = GetReverseAxis(blockDimension);
+      break;
+    default:
+      MOZ_CRASH("Unexpected computed value for 'flex-flow' property");
+  }
+
+  // Determine cross axis:
+  // (This is set up so that a bogus |flexDirection| value will
+  // give us blockDimension.
+  if (flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN ||
+      flexDirection == NS_STYLE_FLEX_DIRECTION_COLUMN_REVERSE) {
+    mCrossAxis = inlineDimension;
+  } else {
+    mCrossAxis = blockDimension;
+  }
+
+  // "flex-wrap: wrap-reverse" reverses our cross axis.
+  if (pos->mFlexWrap == NS_STYLE_FLEX_WRAP_WRAP_REVERSE) {
+    mCrossAxis = GetReverseAxis(mCrossAxis);
+  }
+
+  // Master switch to enable/disable bug 983427's code for reversing our axes
+  // and reversing some logic, to avoid reflowing children in bottom-to-top
+  // order. (This switch can be removed eventually, but for now, it allows
+  // this special-case code path to be compared against the normal code path.)
+  static bool sPreventBottomToTopChildOrdering = true;
+
+  if (sPreventBottomToTopChildOrdering) {
+    // If either axis is bottom-to-top, we flip both axes (and set a flag
+    // so that we can flip some logic to make the reversal transparent).
+    if (eAxis_BT == mMainAxis || eAxis_BT == mCrossAxis) {
+      mMainAxis = GetReverseAxis(mMainAxis);
+      mCrossAxis = GetReverseAxis(mCrossAxis);
+      mAreAxesInternallyReversed = true;
+    }
+  }
+
+  MOZ_ASSERT(IsAxisHorizontal(mMainAxis) != IsAxisHorizontal(mCrossAxis),
+             "main & cross axes should be in different dimensions");
+}
+
+// Allocates a new FlexLine, adds it to the given LinkedList (at the front or
+// back depending on aShouldInsertAtFront), and returns a pointer to it.
+static FlexLine*
+AddNewFlexLineToList(LinkedList<FlexLine>& aLines,
+                     bool aShouldInsertAtFront)
+{
+  FlexLine* newLine = new FlexLine();
+  if (aShouldInsertAtFront) {
+    aLines.insertFront(newLine);
+  } else {
+    aLines.insertBack(newLine);
+  }
+  return newLine;
+}
+
+nsresult
+nsFlexContainerFrame::GenerateFlexLines(
+  nsPresContext* aPresContext,
+  const nsHTMLReflowState& aReflowState,
+  nscoord aContentBoxMainSize,
+  nscoord aAvailableHeightForContent,
+  const nsTArray<StrutInfo>& aStruts,
+  const FlexboxAxisTracker& aAxisTracker,
+  LinkedList<FlexLine>& aLines)
+{
+  MOZ_ASSERT(aLines.isEmpty(), "Expecting outparam to start out empty");
+
+  const bool isSingleLine =
+    NS_STYLE_FLEX_WRAP_NOWRAP == aReflowState.mStylePosition->mFlexWrap;
+
+  // If we're transparently reversing axes, then we'll need to link up our
+  // FlexItems and FlexLines in the reverse order, so that the rest of flex
+  // layout (with flipped axes) will still produce the correct result.
+  // Here, we declare a convenience bool that we'll pass when adding a new
+  // FlexLine or FlexItem, to make us insert it at the beginning of its list
+  // (so the list ends up reversed).
+  const bool shouldInsertAtFront = aAxisTracker.AreAxesInternallyReversed();
+
+  // We have at least one FlexLine. Even an empty flex container has a single
+  // (empty) flex line.
+  FlexLine* curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+
+  nscoord wrapThreshold;
+  if (isSingleLine) {
+    // Not wrapping. Set threshold to sentinel value that tells us not to wrap.
+    wrapThreshold = NS_UNCONSTRAINEDSIZE;
+  } else {
+    // Wrapping! Set wrap threshold to flex container's content-box main-size.
+    wrapThreshold = aContentBoxMainSize;
+
+    // If the flex container doesn't have a definite content-box main-size
+    // (e.g. if we're 'height:auto'), make sure we at least wrap when we hit
+    // its max main-size.
+    if (wrapThreshold == NS_UNCONSTRAINEDSIZE) {
+      const nscoord flexContainerMaxMainSize =
+        GET_MAIN_COMPONENT(aAxisTracker,
+                           aReflowState.ComputedMaxWidth(),
+                           aReflowState.ComputedMaxHeight());
+
+      wrapThreshold = flexContainerMaxMainSize;
+    }
+
+    // Also: if we're vertical and paginating, we may need to wrap sooner
+    // (before we run off the end of the page)
+    if (!IsAxisHorizontal(aAxisTracker.GetMainAxis()) &&
+        aAvailableHeightForContent != NS_UNCONSTRAINEDSIZE) {
+      wrapThreshold = std::min(wrapThreshold, aAvailableHeightForContent);
+    }
+  }
+
+  // Tracks the index of the next strut, in aStruts (and when this hits
+  // aStruts.Length(), that means there are no more struts):
+  uint32_t nextStrutIdx = 0;
+
+  // Overall index of the current flex item in the flex container. (This gets
+  // checked against entries in aStruts.)
+  uint32_t itemIdxInContainer = 0;
+
+  for (nsFrameList::Enumerator e(mFrames); !e.AtEnd(); e.Next()) {
+    nsIFrame* childFrame = e.get();
+
+    // Honor "page-break-before", if we're multi-line and this line isn't empty:
+    if (!isSingleLine && !curLine->IsEmpty() &&
+        childFrame->StyleDisplay()->mBreakBefore) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+
+    nsAutoPtr<FlexItem> item;
+    if (nextStrutIdx < aStruts.Length() &&
+        aStruts[nextStrutIdx].mItemIdx == itemIdxInContainer) {
+
+      // Use the simplified "strut" FlexItem constructor:
+      item = new FlexItem(childFrame, aStruts[nextStrutIdx].mStrutCrossSize);
+      nextStrutIdx++;
+    } else {
+      item = GenerateFlexItemForChild(aPresContext, childFrame,
+                                      aReflowState, aAxisTracker);
+
+      nsresult rv = ResolveFlexItemMaxContentSizing(aPresContext, *item,
+                                                    aReflowState, aAxisTracker);
+      NS_ENSURE_SUCCESS(rv,rv);
+    }
+
+    nscoord itemInnerHypotheticalMainSize = item->GetMainSize();
+    nscoord itemOuterHypotheticalMainSize =
+      item->GetOuterMainSize(aAxisTracker.GetMainAxis());
+
+    // Check if we need to wrap |item| to a new line
+    // (i.e. check if its outer hypothetical main size pushes our line over
+    // the threshold)
+    if (wrapThreshold != NS_UNCONSTRAINEDSIZE &&
+        !curLine->IsEmpty() && // No need to wrap at start of a line.
+        wrapThreshold < (curLine->GetTotalOuterHypotheticalMainSize() +
+                         itemOuterHypotheticalMainSize)) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+
+    // Add item to current flex line (and update the line's bookkeeping about
+    // how large its items collectively are).
+    curLine->AddItem(item.forget(), shouldInsertAtFront,
+                     itemInnerHypotheticalMainSize,
+                     itemOuterHypotheticalMainSize);
+
+    // Honor "page-break-after", if we're multi-line and have more children:
+    if (!isSingleLine && childFrame->GetNextSibling() &&
+        childFrame->StyleDisplay()->mBreakAfter) {
+      curLine = AddNewFlexLineToList(aLines, shouldInsertAtFront);
+    }
+    itemIdxInContainer++;
+  }
+
+  return NS_OK;
+}
+
+// Retrieves the content-box main-size of our flex container from the
+// reflow state (specifically, the main-size of *this continuation* of the
+// flex container).
+nscoord
+nsFlexContainerFrame::GetMainSizeFromReflowState(
+  const nsHTMLReflowState& aReflowState,
+  const FlexboxAxisTracker& aAxisTracker)
+{
+  if (IsAxisHorizontal(aAxisTracker.GetMainAxis())) {
+    // Horizontal case is easy -- our main size is our computed width
+    // (which is already resolved).
+    return aReflowState.ComputedWidth();
+  }
+
+  return GetEffectiveComputedHeight(aReflowState);
+}
+
+// Returns the largest outer hypothetical main-size of any line in |aLines|.
+// (i.e. the hypothetical main-size of the largest line)
+static nscoord
+GetLargestLineMainSize(const FlexLine* aFirstLine)
+{
+  nscoord largestLineOuterSize = 0;
+  for (const FlexLine* line = aFirstLine; line; line = line->getNext()) {
+    largestLineOuterSize = std::max(largestLineOuterSize,
+                                    line->GetTotalOuterHypotheticalMainSize());
+  }
+  return largestLineOuterSize;
+}
+
+// Returns the content-box main-size of our flex container, based on the
+// available height (if appropriate) and the main-sizes of the flex items.
+static nscoord
+ClampFlexContainerMainSize(const nsHTMLReflowState& aReflowState,
+                           const FlexboxAxisTracker& aAxisTracker,
+                           nscoord aUnclampedMainSize,
+                           nscoord aAvailableHeightForContent,
+                           const FlexLine* aFirstLine,
+                           nsReflowStatus& aStatus)
+{
+  MOZ_ASSERT(aFirstLine, "null first line pointer");
+
+  if (IsAxisHorizontal(aAxisTracker.GetMainAxis())) {
+    // Horizontal case is easy -- our main size should already be resolved
+    // before we get a call to Reflow. We don't have to worry about doing
+    // page-breaking or shrinkwrapping in the horizontal axis.
+    return aUnclampedMainSize;
+  }
+
+  if (aUnclampedMainSize != NS_INTRINSICSIZE) {
+    // Vertical case, with fixed height:
+    if (aAvailableHeightForContent == NS_UNCONSTRAINEDSIZE ||
+        aUnclampedMainSize < aAvailableHeightForContent) {
+      // Not in a fragmenting context, OR no need to fragment because we have
+      // more available height than we need. Either way, just use our fixed
+      // height.  (Note that the reflow state has already done the appropriate
+      // min/max-height clamping.)
+      return aUnclampedMainSize;
+    }
+
+    // Fragmenting *and* our fixed height is too tall for available height:
+    // Mark incomplete so we get a next-in-flow, and take up all of the
+    // available height (or the amount of height required by our children, if
+    // that's larger; but of course not more than our own computed height).
+    // XXXdholbert For now, we don't support pushing children to our next
+    // continuation or splitting children, so "amount of height required by
+    // our children" is just the main-size (height) of our longest flex line.
+    NS_FRAME_SET_INCOMPLETE(aStatus);
+    nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);
+
+    if (largestLineOuterSize <= aAvailableHeightForContent) {
+      return aAvailableHeightForContent;
+    }
+    return std::min(aUnclampedMainSize, largestLineOuterSize);
+  }
+
+  // Vertical case, with auto-height:
+  // Resolve auto-height to the largest FlexLine-length, clamped to our
+  // computed min/max main-size properties (min-height & max-height).
+  // XXXdholbert Handle constrained-aAvailableHeightForContent case here.
+  nscoord largestLineOuterSize = GetLargestLineMainSize(aFirstLine);
+  return NS_CSS_MINMAX(largestLineOuterSize,
+                       aReflowState.ComputedMinHeight(),
+                       aReflowState.ComputedMaxHeight());
+}
+
+nscoord
+nsFlexContainerFrame::ComputeCrossSize(const nsHTMLReflowState& aReflowState,
+                                       const FlexboxAxisTracker& aAxisTracker,
+                                       nscoord aSumLineCrossSizes,
+                                       nscoord aAvailableHeightForContent,
+                                       bool* aIsDefinite,
+                                       nsReflowStatus& aStatus)
+{
+  MOZ_ASSERT(aIsDefinite, "outparam pointer must be non-null");
+
+  if (IsAxisHorizontal(aAxisTracker.GetCrossAxis())) {
+    // Cross axis is horizontal: our cross size is our computed width
+    // (which is already resolved).
+    *aIsDefinite = true;
+    return aReflowState.ComputedWidth();
+  }
+
+  nscoord effectiveComputedHeight = GetEffectiveComputedHeight(aReflowState);
+  if (effectiveComputedHeight != NS_INTRINSICSIZE) {
+    // Cross-axis is vertical, and we have a fixed height:
+    *aIsDefinite = true;
+    if (aAvailableHeightForContent == NS_UNCONSTRAINEDSIZE ||
+        effectiveComputedHeight < aAvailableHeightForContent) {
+      // Not in a fragmenting context, OR no need to fragment because we have
+      // more available height than we need. Either way, just use our fixed
+      // height.  (Note that the reflow state has already done the appropriate
+      // min/max-height clamping.)
+      return effectiveComputedHeight;
+    }
+
+    // Fragmenting *and* our fixed height is too tall for available height:
+    // Mark incomplete so we get a next-in-flow, and take up all of the
+    // available height (or the amount of height required by our children, if
+    // that's larger; but of course not more than our own computed height).
+    // XXXdholbert For now, we don't support pushing children to our next
+    // continuation or splitting children, so "amount of height required by
+    // our children" is just our line-height.
+    NS_FRAME_SET_INCOMPLETE(aStatus);
+    if (aSumLineCrossSizes <= aAvailableHeightForContent) {
+      return aAvailableHeightForContent;
+    }
+    return std::min(effectiveComputedHeight, aSumLineCrossSizes);
+  }
+
+  // Cross axis is vertical and we have auto-height: shrink-wrap our line(s),
+  // subject to our min-size / max-size constraints in that (vertical) axis.
+  // XXXdholbert Handle constrained-aAvailableHeightForContent case here.
+  *aIsDefinite = false;
+  return NS_CSS_MINMAX(aSumLineCrossSizes,
+                       aReflowState.ComputedMinHeight(),
+                       aReflowState.ComputedMaxHeight());
+}
+
+void
+FlexLine::PositionItemsInMainAxis(uint8_t aJustifyContent,
+                                  nscoord aContentBoxMainSize,
+                                  const FlexboxAxisTracker& aAxisTracker)
+{
+  MainAxisPositionTracker mainAxisPosnTracker(aAxisTracker, this,
+                                              aJustifyContent,
+                                              aContentBoxMainSize);
+  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    nscoord itemMainBorderBoxSize =
+      item->GetMainSize() +
+      item->GetBorderPaddingSizeInAxis(mainAxisPosnTracker.GetAxis());
+
+    // Resolve any main-axis 'auto' margins on aChild to an actual value.
+    mainAxisPosnTracker.ResolveAutoMarginsInMainAxis(*item);
+
+    // Advance our position tracker to child's upper-left content-box corner,
+    // and use that as its position in the main axis.
+    mainAxisPosnTracker.EnterMargin(item->GetMargin());
+    mainAxisPosnTracker.EnterChildFrame(itemMainBorderBoxSize);
+
+    item->SetMainPosition(mainAxisPosnTracker.GetPosition());
+
+    mainAxisPosnTracker.ExitChildFrame(itemMainBorderBoxSize);
+    mainAxisPosnTracker.ExitMargin(item->GetMargin());
+    mainAxisPosnTracker.TraversePackingSpace();
+  }
+}
+
+// Helper method to take care of children who ASK_FOR_BASELINE, when
+// we need their baseline.
+static void
+ResolveReflowedChildAscent(nsIFrame* aFrame,
+                           nsHTMLReflowMetrics& aChildDesiredSize)
+{
+  if (aChildDesiredSize.TopAscent() == nsHTMLReflowMetrics::ASK_FOR_BASELINE) {
+    // Use GetFirstLineBaseline(), or just GetBaseline() if that fails.
+    nscoord ascent;
+    if (nsLayoutUtils::GetFirstLineBaseline(aFrame, &ascent)) {
+      aChildDesiredSize.SetTopAscent(ascent);
+    } else {
+      aChildDesiredSize.SetTopAscent(aFrame->GetBaseline());
+    }
+  }
+}
+
+/**
+ * Given the flex container's "logical ascent" (i.e. distance from the
+ * flex container's content-box cross-start edge to its baseline), returns
+ * its actual physical ascent value (the distance from the *border-box* top
+ * edge to its baseline).
+ */
+static nscoord
+ComputePhysicalAscentFromLogicalAscent(nscoord aLogicalAscent,
+                                       nscoord aContentBoxCrossSize,
+                                       const nsHTMLReflowState& aReflowState,
+                                       const FlexboxAxisTracker& aAxisTracker)
+{
+  return aReflowState.ComputedPhysicalBorderPadding().top +
+    PhysicalPosFromLogicalPos(aLogicalAscent, aContentBoxCrossSize,
+                              aAxisTracker.GetCrossAxis());
+}
+
+nsresult
+nsFlexContainerFrame::SizeItemInCrossAxis(
+  nsPresContext* aPresContext,
+  const FlexboxAxisTracker& aAxisTracker,
+  nsHTMLReflowState& aChildReflowState,
+  FlexItem& aItem)
+{
+  // In vertical flexbox (with horizontal cross-axis), we can just trust the
+  // reflow state's computed-width as our cross-size. We also don't need to
+  // record the baseline because we'll have converted any "align-self:baseline"
+  // items to be "align-self:flex-start" in the FlexItem constructor.
+  // FIXME: Once we support writing-mode (vertical text), we will be able to
+  // have baseline-aligned items in a vertical flexbox, and we'll need to
+  // record baseline information here.
+  if (IsAxisHorizontal(aAxisTracker.GetCrossAxis())) {
+    MOZ_ASSERT(aItem.GetAlignSelf() != NS_STYLE_ALIGN_ITEMS_BASELINE,
+               "In vert flex container, we depend on FlexItem constructor to "
+               "convert 'align-self: baseline' to 'align-self: flex-start'");
+    aItem.SetCrossSize(aChildReflowState.ComputedWidth());
+    return NS_OK;
+  }
+
+  MOZ_ASSERT(!aItem.HadMeasuringReflow(),
+             "We shouldn't need more than one measuring reflow");
+
+  if (aItem.GetAlignSelf() == NS_STYLE_ALIGN_ITEMS_STRETCH) {
+    // This item's got "align-self: stretch", so we probably imposed a
+    // stretched computed height on it during its previous reflow. We're
+    // not imposing that height for *this* measuring reflow, so we need to
+    // tell it to treat this reflow as a vertical resize (regardless of
+    // whether any of its ancestors are being resized).
+    aChildReflowState.mFlags.mVResize = true;
+  }
+  nsHTMLReflowMetrics childDesiredSize(aChildReflowState);
+  nsReflowStatus childReflowStatus;
+  const uint32_t flags = NS_FRAME_NO_MOVE_FRAME;
+  nsresult rv = ReflowChild(aItem.Frame(), aPresContext,
+                            childDesiredSize, aChildReflowState,
+                            0, 0, flags, childReflowStatus);
+  aItem.SetHadMeasuringReflow();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // XXXdholbert Once we do pagination / splitting, we'll need to actually
+  // handle incomplete childReflowStatuses. But for now, we give our kids
+  // unconstrained available height, which means they should always complete.
+  MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+             "We gave flex item unconstrained available height, so it "
+             "should be complete");
+
+  // Tell the child we're done with its initial reflow.
+  // (Necessary for e.g. GetBaseline() to work below w/out asserting)
+  rv = FinishReflowChild(aItem.Frame(), aPresContext,
+                         childDesiredSize, &aChildReflowState, 0, 0, flags);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Save the sizing info that we learned from this reflow
+  // -----------------------------------------------------
+
+  // Tentatively store the child's desired content-box cross-size.
+  // Note that childDesiredSize is the border-box size, so we have to
+  // subtract border & padding to get the content-box size.
+  // (Note that at this point in the code, we know our cross axis is vertical,
+  // so we don't bother with making aAxisTracker pick the cross-axis component
+  // for us.)
+  nscoord crossAxisBorderPadding = aItem.GetBorderPadding().TopBottom();
+  if (childDesiredSize.Height() < crossAxisBorderPadding) {
+    // Child's requested size isn't large enough for its border/padding!
+    // This is OK for the trivial nsFrame::Reflow() impl, but other frame
+    // classes should know better. So, if we get here, the child had better be
+    // an instance of nsFrame (i.e. it should return null from GetType()).
+    // XXXdholbert Once we've fixed bug 765861, we should upgrade this to an
+    // assertion that trivially passes if bug 765861's flag has been flipped.
+    NS_WARN_IF_FALSE(!aItem.Frame()->GetType(),
+                     "Child should at least request space for border/padding");
+    aItem.SetCrossSize(0);
+  } else {
+    // (normal case)
+    aItem.SetCrossSize(childDesiredSize.Height() - crossAxisBorderPadding);
+  }
+
+  // If we need to do baseline-alignment, store the child's ascent.
+  if (aItem.GetAlignSelf() == NS_STYLE_ALIGN_ITEMS_BASELINE) {
+    ResolveReflowedChildAscent(aItem.Frame(), childDesiredSize);
+    aItem.SetAscent(childDesiredSize.TopAscent());
+  }
+
+  return NS_OK;
+}
+
+void
+FlexLine::PositionItemsInCrossAxis(nscoord aLineStartPosition,
+                                   const FlexboxAxisTracker& aAxisTracker)
+{
+  SingleLineCrossAxisPositionTracker lineCrossAxisPosnTracker(aAxisTracker);
+
+  for (FlexItem* item = mItems.getFirst(); item; item = item->getNext()) {
+    // First, stretch the item's cross size (if appropriate), and resolve any
+    // auto margins in this axis.
+    item->ResolveStretchedCrossSize(mLineCrossSize, aAxisTracker);
+    lineCrossAxisPosnTracker.ResolveAutoMarginsInCrossAxis(*this, *item);
+
+    // Compute the cross-axis position of this item
+    nscoord itemCrossBorderBoxSize =
+      item->GetCrossSize() +
+      item->GetBorderPaddingSizeInAxis(aAxisTracker.GetCrossAxis());
+    lineCrossAxisPosnTracker.EnterAlignPackingSpace(*this, *item, aAxisTracker);
+    lineCrossAxisPosnTracker.EnterMargin(item->GetMargin());
+    lineCrossAxisPosnTracker.EnterChildFrame(itemCrossBorderBoxSize);
+
+    item->SetCrossPosition(aLineStartPosition +
+                           lineCrossAxisPosnTracker.GetPosition());
+
+    // Back out to cross-axis edge of the line.
+    lineCrossAxisPosnTracker.ResetPosition();
+  }
+}
+
+nsresult
+nsFlexContainerFrame::Reflow(nsPresContext*           aPresContext,
+                             nsHTMLReflowMetrics&     aDesiredSize,
+                             const nsHTMLReflowState& aReflowState,
+                             nsReflowStatus&          aStatus)
+{
+  DO_GLOBAL_REFLOW_COUNT("nsFlexContainerFrame");
+  DISPLAY_REFLOW(aPresContext, this, aReflowState, aDesiredSize, aStatus);
+  PR_LOG(GetFlexContainerLog(), PR_LOG_DEBUG,
+         ("Reflow() for nsFlexContainerFrame %p\n", this));
+
+  if (IsFrameTreeTooDeep(aReflowState, aDesiredSize, aStatus)) {
+    return NS_OK;
+  }
+
+  // We (and our children) can only depend on our ancestor's height if we have
+  // a percent-height, or if we're positioned and we have "top" and "bottom"
+  // set and have height:auto.  (There are actually other cases, too -- e.g. if
+  // our parent is itself a vertical flex container and we're flexible -- but
+  // we'll let our ancestors handle those sorts of cases.)
+  const nsStylePosition* stylePos = StylePosition();
+  if (stylePos->mHeight.HasPercent() ||
+      (StyleDisplay()->IsAbsolutelyPositionedStyle() &&
+       eStyleUnit_Auto == stylePos->mHeight.GetUnit() &&
+       eStyleUnit_Auto != stylePos->mOffset.GetTopUnit() &&
+       eStyleUnit_Auto != stylePos->mOffset.GetBottomUnit())) {
+    AddStateBits(NS_FRAME_CONTAINS_RELATIVE_HEIGHT);
+  }
+
+#ifdef DEBUG
+  SanityCheckAnonymousFlexItems();
+#endif // DEBUG
+
+  // If we've never reordered our children, then we can trust that they're
+  // already in DOM-order, and we only need to consider their "order" property
+  // when checking them for sortedness & sorting them.
+  //
+  // After we actually sort them, though, we can't trust that they're in DOM
+  // order anymore.  So, from that point on, our sort & sorted-order-checking
+  // operations need to use a fancier LEQ function that also takes DOM order
+  // into account, so that we can honor the spec's requirement that frames w/
+  // equal "order" values are laid out in DOM order.
+
+  if (!HasAnyStateBits(NS_STATE_FLEX_CHILDREN_REORDERED)) {
+    if (SortChildrenIfNeeded<IsOrderLEQ>()) {
+      AddStateBits(NS_STATE_FLEX_CHILDREN_REORDERED);
+    }
+  } else {
+    SortChildrenIfNeeded<IsOrderLEQWithDOMFallback>();
+  }
+
+  const FlexboxAxisTracker axisTracker(this);
+
+  // If we're being fragmented into a constrained height, subtract off
+  // borderpadding-top from it, to get the available height for our
+  // content box. (Don't subtract if we're skipping top border/padding,
+  // though.)
+  nscoord availableHeightForContent = aReflowState.AvailableHeight();
+  if (availableHeightForContent != NS_UNCONSTRAINEDSIZE &&
+      !(GetSkipSides() & (1 << NS_SIDE_TOP))) {
+    availableHeightForContent -= aReflowState.ComputedPhysicalBorderPadding().top;
+    // (Don't let that push availableHeightForContent below zero, though):
+    availableHeightForContent = std::max(availableHeightForContent, 0);
+  }
+
+  nscoord contentBoxMainSize = GetMainSizeFromReflowState(aReflowState,
+                                                          axisTracker);
+
+  nsAutoTArray<StrutInfo, 1> struts;
+  nsresult rv = DoFlexLayout(aPresContext, aDesiredSize, aReflowState, aStatus,
+                             contentBoxMainSize, availableHeightForContent,
+                             struts, axisTracker);
+
+  if (NS_SUCCEEDED(rv) && !struts.IsEmpty()) {
+    // We're restarting flex layout, with new knowledge of collapsed items.
+    rv = DoFlexLayout(aPresContext, aDesiredSize, aReflowState, aStatus,
+                      contentBoxMainSize, availableHeightForContent,
+                      struts, axisTracker);
+  }
+
+  return rv;
+}
+
+// RAII class to clean up a list of FlexLines.
+// Specifically, this removes each line from the list, deletes all the
+// FlexItems in its list, and deletes the FlexLine.
+class MOZ_STACK_CLASS AutoFlexLineListClearer
+{
+public:
+  AutoFlexLineListClearer(LinkedList<FlexLine>& aLines
+                          MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+  : mLines(aLines)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+  }
+
+  ~AutoFlexLineListClearer()
+  {
+    while (FlexLine* line = mLines.popFirst()) {
+      while (FlexItem* item = line->mItems.popFirst()) {
+        delete item;
+      }
+      delete line;
+    }
+  }
+
+private:
+  LinkedList<FlexLine>& mLines;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+nsresult
+nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
+                                   nsHTMLReflowMetrics&     aDesiredSize,
+                                   const nsHTMLReflowState& aReflowState,
+                                   nsReflowStatus&          aStatus,
+                                   nscoord aContentBoxMainSize,
+                                   nscoord aAvailableHeightForContent,
+                                   nsTArray<StrutInfo>& aStruts,
+                                   const FlexboxAxisTracker& aAxisTracker)
+{
+  aStatus = NS_FRAME_COMPLETE;
+
+  LinkedList<FlexLine> lines;
+  AutoFlexLineListClearer cleanupLines(lines);
+
+  nsresult rv = GenerateFlexLines(aPresContext, aReflowState,
+                                  aContentBoxMainSize,
+                                  aAvailableHeightForContent,
+                                  aStruts, aAxisTracker, lines);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  aContentBoxMainSize =
+    ClampFlexContainerMainSize(aReflowState, aAxisTracker,
+                               aContentBoxMainSize, aAvailableHeightForContent,
+                               lines.getFirst(), aStatus);
+
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    line->ResolveFlexibleLengths(aContentBoxMainSize);
+  }
+
+  // Cross Size Determination - Flexbox spec section 9.4
+  // ===================================================
+  // Calculate the hypothetical cross size of each item:
+  nscoord sumLineCrossSizes = 0;
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    for (FlexItem* item = line->GetFirstItem(); item; item = item->getNext()) {
+      // (If the item's already been stretched, or it's a strut, then it
+      // already knows its cross size.  Don't bother trying to recalculate it.)
+      if (!item->IsStretched() && !item->IsStrut()) {
+        nsHTMLReflowState childReflowState(aPresContext, aReflowState,
+                                           item->Frame(),
+                                           nsSize(aReflowState.ComputedWidth(),
+                                                  NS_UNCONSTRAINEDSIZE));
+        // Override computed main-size
+        if (IsAxisHorizontal(aAxisTracker.GetMainAxis())) {
+          childReflowState.SetComputedWidth(item->GetMainSize());
+        } else {
+          childReflowState.SetComputedHeight(item->GetMainSize());
+        }
+
+        nsresult rv = SizeItemInCrossAxis(aPresContext, aAxisTracker,
+                                          childReflowState, *item);
+        NS_ENSURE_SUCCESS(rv, rv);
+      }
+    }
+    // Now that we've finished with this line's items, size the line itself:
+    line->ComputeCrossSizeAndBaseline(aAxisTracker);
+    sumLineCrossSizes += line->GetLineCrossSize();
+  }
+
+  bool isCrossSizeDefinite;
+  const nscoord contentBoxCrossSize =
+    ComputeCrossSize(aReflowState, aAxisTracker, sumLineCrossSizes,
+                     aAvailableHeightForContent, &isCrossSizeDefinite, aStatus);
+
+  // Set up state for cross-axis alignment, at a high level (outside the
+  // scope of a particular flex line)
+  CrossAxisPositionTracker
+    crossAxisPosnTracker(lines.getFirst(),
+                         aReflowState.mStylePosition->mAlignContent,
+                         contentBoxCrossSize, isCrossSizeDefinite,
+                         aAxisTracker);
+
+  // Now that we know the cross size of each line (including
+  // "align-content:stretch" adjustments, from the CrossAxisPositionTracker
+  // constructor), we can create struts for any flex items with
+  // "visibility: collapse" (and restart flex layout).
+  if (aStruts.IsEmpty()) { // (Don't make struts if we already did)
+    BuildStrutInfoFromCollapsedItems(lines.getFirst(), aStruts);
+    if (!aStruts.IsEmpty()) {
+      // Restart flex layout, using our struts.
+      return NS_OK;
+    }
+  }
+
+  // If the container should derive its baseline from the first FlexLine,
+  // do that here (while crossAxisPosnTracker is conveniently pointing
+  // at the cross-start edge of that line, which the line's baseline offset is
+  // measured from):
+  nscoord flexContainerAscent;
+  if (!aAxisTracker.AreAxesInternallyReversed()) {
+    nscoord firstLineBaselineOffset = lines.getFirst()->GetBaselineOffset();
+    if (firstLineBaselineOffset == nscoord_MIN) {
+      // No baseline-aligned items in line. Use sentinel value to prompt us to
+      // get baseline from the first FlexItem after we've reflowed it.
+      flexContainerAscent = nscoord_MIN;
+    } else  {
+      flexContainerAscent =
+        ComputePhysicalAscentFromLogicalAscent(
+          crossAxisPosnTracker.GetPosition() + firstLineBaselineOffset,
+          contentBoxCrossSize, aReflowState, aAxisTracker);
+    }
+  }
+
+  for (FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+
+    // Main-Axis Alignment - Flexbox spec section 9.5
+    // ==============================================
+    line->PositionItemsInMainAxis(aReflowState.mStylePosition->mJustifyContent,
+                                  aContentBoxMainSize,
+                                  aAxisTracker);
+
+    // Cross-Axis Alignment - Flexbox spec section 9.6
+    // ===============================================
+    line->PositionItemsInCrossAxis(crossAxisPosnTracker.GetPosition(),
+                                   aAxisTracker);
+    crossAxisPosnTracker.TraverseLine(*line);
+    crossAxisPosnTracker.TraversePackingSpace();
+  }
+
+  // If the container should derive its baseline from the last FlexLine,
+  // do that here (while crossAxisPosnTracker is conveniently pointing
+  // at the cross-end edge of that line, which the line's baseline offset is
+  // measured from):
+  if (aAxisTracker.AreAxesInternallyReversed()) {
+    nscoord lastLineBaselineOffset = lines.getLast()->GetBaselineOffset();
+    if (lastLineBaselineOffset == nscoord_MIN) {
+      // No baseline-aligned items in line. Use sentinel value to prompt us to
+      // get baseline from the last FlexItem after we've reflowed it.
+      flexContainerAscent = nscoord_MIN;
+    } else {
+      flexContainerAscent =
+        ComputePhysicalAscentFromLogicalAscent(
+          crossAxisPosnTracker.GetPosition() - lastLineBaselineOffset,
+          contentBoxCrossSize, aReflowState, aAxisTracker);
+    }
+  }
+
+  // Before giving each child a final reflow, calculate the origin of the
+  // flex container's content box (with respect to its border-box), so that
+  // we can compute our flex item's final positions.
+  nsMargin containerBorderPadding(aReflowState.ComputedPhysicalBorderPadding());
+  ApplySkipSides(containerBorderPadding, &aReflowState);
+  const nsPoint containerContentBoxOrigin(containerBorderPadding.left,
+                                          containerBorderPadding.top);
+
+  // FINAL REFLOW: Give each child frame another chance to reflow, now that
+  // we know its final size and position.
+  for (const FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
+    for (const FlexItem* item = line->GetFirstItem(); item;
+         item = item->getNext()) {
+      nsPoint physicalPosn = aAxisTracker.PhysicalPointFromLogicalPoint(
+                               item->GetMainPosition(),
+                               item->GetCrossPosition(),
+                               aContentBoxMainSize,
+                               contentBoxCrossSize);
+      // Adjust physicalPosn to be relative to the container's border-box
+      // (i.e. its frame rect), instead of the container's content-box:
+      physicalPosn += containerContentBoxOrigin;
+
+      nsHTMLReflowState childReflowState(aPresContext, aReflowState,
+                                         item->Frame(),
+                                         nsSize(aReflowState.ComputedWidth(),
+                                                NS_UNCONSTRAINEDSIZE));
+
+      // Keep track of whether we've overriden the child's computed height
+      // and/or width, so we can set its resize flags accordingly.
+      bool didOverrideComputedWidth = false;
+      bool didOverrideComputedHeight = false;
+
+      // Override computed main-size
+      if (IsAxisHorizontal(aAxisTracker.GetMainAxis())) {
+        childReflowState.SetComputedWidth(item->GetMainSize());
+        didOverrideComputedWidth = true;
+      } else {
+        childReflowState.SetComputedHeight(item->GetMainSize());
+        didOverrideComputedHeight = true;
+      }
+
+      // Override reflow state's computed cross-size, for stretched items.
+      if (item->IsStretched()) {
+        MOZ_ASSERT(item->GetAlignSelf() == NS_STYLE_ALIGN_ITEMS_STRETCH,
+                   "stretched item w/o 'align-self: stretch'?");
+        if (IsAxisHorizontal(aAxisTracker.GetCrossAxis())) {
+          childReflowState.SetComputedWidth(item->GetCrossSize());
+          didOverrideComputedWidth = true;
+        } else {
+          // If this item's height is stretched, it's a relative height.
+          item->Frame()->AddStateBits(NS_FRAME_CONTAINS_RELATIVE_HEIGHT);
+          childReflowState.SetComputedHeight(item->GetCrossSize());
+          didOverrideComputedHeight = true;
+        }
+      }
+
+      // XXXdholbert Might need to actually set the correct margins in the
+      // reflow state at some point, so that they can be saved on the frame for
+      // UsedMarginProperty().  Maybe doesn't matter though...?
+
+      // If we're overriding the computed width or height, *and* we had an
+      // earlier "measuring" reflow, then this upcoming reflow needs to be
+      // treated as a resize.
+      if (item->HadMeasuringReflow()) {
+        if (didOverrideComputedWidth) {
+          // (This is somewhat redundant, since the reflow state already
+          // sets mHResize whenever our computed width has changed since the
+          // previous reflow. Still, it's nice for symmetry, and it may become
+          // necessary once we support orthogonal flows.)
+          childReflowState.mFlags.mHResize = true;
+        }
+        if (didOverrideComputedHeight) {
+          childReflowState.mFlags.mVResize = true;
+        }
+      }
+      // NOTE: Be very careful about doing anything else with childReflowState
+      // after this point, because some of its methods (e.g. SetComputedWidth)
+      // internally call InitResizeFlags and stomp on mVResize & mHResize.
+
+      nsHTMLReflowMetrics childDesiredSize(childReflowState);
+      nsReflowStatus childReflowStatus;
+      nsresult rv = ReflowChild(item->Frame(), aPresContext,
+                                childDesiredSize, childReflowState,
+                                physicalPosn.x, physicalPosn.y,
+                                0, childReflowStatus);
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      // XXXdholbert Once we do pagination / splitting, we'll need to actually
+      // handle incomplete childReflowStatuses. But for now, we give our kids
+      // unconstrained available height, which means they should always
+      // complete.
+      MOZ_ASSERT(NS_FRAME_IS_COMPLETE(childReflowStatus),
+                 "We gave flex item unconstrained available height, so it "
+                 "should be complete");
+
+      childReflowState.ApplyRelativePositioning(&physicalPosn);
+
+      rv = FinishReflowChild(item->Frame(), aPresContext,
+                             childDesiredSize, &childReflowState,
+                             physicalPosn.x, physicalPosn.y, 0);
+      NS_ENSURE_SUCCESS(rv, rv);
+
+      // If this is our first child and we haven't established a baseline for
+      // the container yet (i.e. if we don't have 'align-self: baseline' on any
+      // children), then use this child's baseline as the container's baseline.
+      if (item->Frame() == mFrames.FirstChild() &&
+          flexContainerAscent == nscoord_MIN) {
+        ResolveReflowedChildAscent(item->Frame(), childDesiredSize);
+
+        // (We use GetNormalPosition() instead of physicalPosn because we don't
+        // want relative positioning on the child to affect the baseline that we
+        // read from it).
+        flexContainerAscent = item->Frame()->GetNormalPosition().y +
+          childDesiredSize.TopAscent();
+      }
+    }
+  }
+
+  nsSize desiredContentBoxSize =
+    aAxisTracker.PhysicalSizeFromLogicalSizes(aContentBoxMainSize,
+                                              contentBoxCrossSize);
+
+  aDesiredSize.Width() = desiredContentBoxSize.width +
+    containerBorderPadding.LeftRight();
+  // Does *NOT* include bottom border/padding yet (we add that a bit lower down)
+  aDesiredSize.Height() = desiredContentBoxSize.height +
+    containerBorderPadding.top;
+
+  if (flexContainerAscent == nscoord_MIN) {
+    // Still don't have our baseline set -- this happens if we have no
+    // children (or if our children are huge enough that they have nscoord_MIN
+    // as their baseline... in which case, we'll use the wrong baseline, but no
+    // big deal)
+    NS_WARN_IF_FALSE(lines.getFirst()->IsEmpty(),
+                     "Have flex items but didn't get an ascent - that's odd "
+                     "(or there are just gigantic sizes involved)");
+    // Per spec, just use the bottom of content-box.
+    flexContainerAscent = aDesiredSize.Height();
+  }
+  aDesiredSize.SetTopAscent(flexContainerAscent);
+
+  // Now: If we're complete, add bottom border/padding to desired height
+  // (unless that pushes us over available height, in which case we become
+  // incomplete (unless we already weren't asking for any height, in which case
+  // we stay complete to avoid looping forever)).
+  // NOTE: If we're auto-height, we allow our bottom border/padding to push us
+  // over the available height without requesting a continuation, for
+  // consistency with the behavior of "display:block" elements.
+  if (NS_FRAME_IS_COMPLETE(aStatus)) {
+    // NOTE: We can't use containerBorderPadding.bottom for this, because if
+    // we're auto-height, ApplySkipSides will have zeroed it (because it
+    // assumed we might get a continuation). We have the correct value in
+    // aReflowState.ComputedPhyiscalBorderPadding().bottom, though, so we use that.
+    nscoord desiredHeightWithBottomBP =
+      aDesiredSize.Height() + aReflowState.ComputedPhysicalBorderPadding().bottom;
+
+    if (aReflowState.AvailableHeight() == NS_UNCONSTRAINEDSIZE ||
+        aDesiredSize.Height() == 0 ||
+        desiredHeightWithBottomBP <= aReflowState.AvailableHeight() ||
+        aReflowState.ComputedHeight() == NS_INTRINSICSIZE) {
+      // Update desired height to include bottom border/padding
+      aDesiredSize.Height() = desiredHeightWithBottomBP;
+    } else {
+      // We couldn't fit bottom border/padding, so we'll need a continuation.
+      NS_FRAME_SET_INCOMPLETE(aStatus);
+    }
+  }
+
+  // Overflow area = union(my overflow area, kids' overflow areas)
+  aDesiredSize.SetOverflowAreasToDesiredBounds();
+  for (nsFrameList::Enumerator e(mFrames); !e.AtEnd(); e.Next()) {
+    ConsiderChildOverflow(aDesiredSize.mOverflowAreas, e.get());
+  }
+
+  FinishReflowWithAbsoluteFrames(aPresContext, aDesiredSize,
+                                 aReflowState, aStatus);
+
+  NS_FRAME_SET_TRUNCATION(aStatus, aReflowState, aDesiredSize)
+  return NS_OK;
+}
+
+/* virtual */ nscoord
+nsFlexContainerFrame::GetMinWidth(nsRenderingContext* aRenderingContext)
+{
+  nscoord minWidth = 0;
+  DISPLAY_MIN_WIDTH(this, minWidth);
+
+  FlexboxAxisTracker axisTracker(this);
+
+  for (nsFrameList::Enumerator e(mFrames); !e.AtEnd(); e.Next()) {
+    nscoord childMinWidth =
+      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, e.get(),
+                                           nsLayoutUtils::MIN_WIDTH);
+    // For a horizontal single-line flex container, the intrinsic min width is
+    // the sum of its items' min widths.
+    // For a vertical flex container, or for a multi-line horizontal flex
+    // container, the intrinsic min width is the max of its items' min widths.
+    if (IsAxisHorizontal(axisTracker.GetMainAxis()) &&
+        NS_STYLE_FLEX_WRAP_NOWRAP == StylePosition()->mFlexWrap) {
+      minWidth += childMinWidth;
+    } else {
+      minWidth = std::max(minWidth, childMinWidth);
+    }
+  }
+  return minWidth;
+}
+
+/* virtual */ nscoord
+nsFlexContainerFrame::GetPrefWidth(nsRenderingContext* aRenderingContext)
+{
+  nscoord prefWidth = 0;
+  DISPLAY_PREF_WIDTH(this, prefWidth);
+
+  // XXXdholbert Optimization: We could cache our intrinsic widths like
+  // nsBlockFrame does (and return it early from this function if it's set).
+  // Whenever anything happens that might change it, set it to
+  // NS_INTRINSIC_WIDTH_UNKNOWN (like nsBlockFrame::MarkIntrinsicWidthsDirty
+  // does)
+  FlexboxAxisTracker axisTracker(this);
+
+  for (nsFrameList::Enumerator e(mFrames); !e.AtEnd(); e.Next()) {
+    nscoord childPrefWidth =
+      nsLayoutUtils::IntrinsicForContainer(aRenderingContext, e.get(),
+                                           nsLayoutUtils::PREF_WIDTH);
+    if (IsAxisHorizontal(axisTracker.GetMainAxis())) {
+      prefWidth += childPrefWidth;
+    } else {
+      prefWidth = std::max(prefWidth, childPrefWidth);
+    }
+  }
+  return prefWidth;
+}