diff -r 000000000000 -r 6474c204b198 layout/generic/nsBlockReflowContext.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/generic/nsBlockReflowContext.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+// vim:cindent:ts=2:et:sw=2:
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* class that a parent frame uses to reflow a block frame */
+
+#ifndef nsBlockReflowContext_h___
+#define nsBlockReflowContext_h___
+
+#include "nsIFrame.h"
+#include "nsHTMLReflowMetrics.h"
+
+class nsBlockReflowState;
+struct nsHTMLReflowState;
+class nsLineBox;
+class nsPresContext;
+class nsLineLayout;
+struct nsBlockHorizontalAlign;
+
+/**
+ * An encapsulation of the state and algorithm for reflowing block frames.
+ */
+class nsBlockReflowContext {
+public:
+  nsBlockReflowContext(nsPresContext* aPresContext,
+                       const nsHTMLReflowState& aParentRS);
+  ~nsBlockReflowContext() { }
+
+  nsresult ReflowBlock(const nsRect&       aSpace,
+                       bool                aApplyTopMargin,
+                       nsCollapsingMargin& aPrevMargin,
+                       nscoord             aClearance,
+                       bool                aIsAdjacentWithTop,
+                       nsLineBox*          aLine,
+                       nsHTMLReflowState&  aReflowState,
+                       nsReflowStatus&     aReflowStatus,
+                       nsBlockReflowState& aState);
+
+  bool PlaceBlock(const nsHTMLReflowState& aReflowState,
+                  bool                     aForceFit,
+                  nsLineBox*               aLine,
+                  nsCollapsingMargin&      aBottomMarginResult /* out */,
+                  nsOverflowAreas&         aOverflowAreas,
+                  nsReflowStatus           aReflowStatus,
+                  nscoord                  aContainerWidth);
+
+  nsCollapsingMargin& GetCarriedOutBottomMargin() {
+    return mMetrics.mCarriedOutBottomMargin;
+  }
+
+  nscoord GetTopMargin() const {
+    return mTopMargin.get();
+  }
+
+  const nsHTMLReflowMetrics& GetMetrics() const {
+    return mMetrics;
+  }
+
+  /**
+   * Computes the collapsed top margin for a block whose reflow state is in aRS.
+   * The computed margin is added into aMargin.
+   * If aClearanceFrame is null then this is the first optimistic pass which shall assume
+   * that no frames have clearance, and we clear the HasClearance on all frames encountered.
+   * If non-null, this is the second pass and
+   * the caller has decided aClearanceFrame needs clearance (and we will
+   * therefore stop collapsing there); also, this function is responsible for marking
+   * it with SetHasClearance.
+   * If in the optimistic pass any frame is encountered that might possibly need
+   * clearance (i.e., if we really needed the optimism assumption) then we set aMayNeedRetry
+   * to true.
+   * We return true if we changed the clearance state of any line and marked it dirty.
+   */
+  static bool ComputeCollapsedTopMargin(const nsHTMLReflowState& aRS,
+                                          nsCollapsingMargin* aMargin, nsIFrame* aClearanceFrame,
+                                          bool* aMayNeedRetry, bool* aIsEmpty = nullptr);
+
+protected:
+  nsPresContext* mPresContext;
+  const nsHTMLReflowState& mOuterReflowState;
+
+  nsIFrame* mFrame;
+  nsRect mSpace;
+
+  nscoord mX, mY;
+  nsHTMLReflowMetrics mMetrics;
+  nsCollapsingMargin mTopMargin;
+};
+
+#endif /* nsBlockReflowContext_h___ */