diff -r 000000000000 -r 6474c204b198 widget/windows/nsScreenManagerWin.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/widget/windows/nsScreenManagerWin.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,170 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 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/. */
+
+#include "nsScreenManagerWin.h"
+#include "nsScreenWin.h"
+#include "gfxWindowsPlatform.h"
+#include "nsIWidget.h"
+
+
+BOOL CALLBACK CountMonitors ( HMONITOR, HDC, LPRECT, LPARAM ioCount ) ;
+
+nsScreenManagerWin :: nsScreenManagerWin ( )
+  : mNumberOfScreens(0)
+{
+  // nothing to do. I guess we could cache a bunch of information
+  // here, but we want to ask the device at runtime in case anything
+  // has changed.
+}
+
+
+nsScreenManagerWin :: ~nsScreenManagerWin()
+{
+}
+
+
+// addref, release, QI
+NS_IMPL_ISUPPORTS(nsScreenManagerWin, nsIScreenManager)
+
+
+//
+// CreateNewScreenObject
+//
+// Utility routine. Creates a new screen object from the given device handle
+//
+// NOTE: For this "single-monitor" impl, we just always return the cached primary
+//        screen. This should change when a multi-monitor impl is done.
+//
+nsIScreen* 
+nsScreenManagerWin :: CreateNewScreenObject ( HMONITOR inScreen )
+{
+  nsIScreen* retScreen = nullptr;
+  
+  // look through our screen list, hoping to find it. If it's not there,
+  // add it and return the new one.
+  for ( unsigned i = 0; i < mScreenList.Length(); ++i ) {
+    ScreenListItem& curr = mScreenList[i];
+    if ( inScreen == curr.mMon ) {
+      NS_IF_ADDREF(retScreen = curr.mScreen.get());
+      return retScreen;
+    }
+  } // for each screen.
+ 
+  retScreen = new nsScreenWin(inScreen);
+  mScreenList.AppendElement ( ScreenListItem ( inScreen, retScreen ) );
+
+  NS_IF_ADDREF(retScreen);
+  return retScreen;
+}
+
+
+//
+// ScreenForRect 
+//
+// Returns the screen that contains the rectangle. If the rect overlaps
+// multiple screens, it picks the screen with the greatest area of intersection.
+//
+// The coordinates are in pixels (not twips) and in logical screen coordinates.
+//
+NS_IMETHODIMP
+nsScreenManagerWin :: ScreenForRect ( int32_t inLeft, int32_t inTop, int32_t inWidth, int32_t inHeight,
+                                        nsIScreen **outScreen )
+{
+  if ( !(inWidth || inHeight) ) {
+    NS_WARNING ( "trying to find screen for sizeless window, using primary monitor" );
+    *outScreen = CreateNewScreenObject ( nullptr );    // addrefs
+    return NS_OK;
+  }
+
+  // convert coordinates from logical to device pixels for MonitorFromRect
+  double dpiScale = nsIWidget::DefaultScaleOverride();
+  if (dpiScale <= 0.0) {
+    dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale(); 
+  }
+  RECT globalWindowBounds = {
+    NSToIntRound(dpiScale * inLeft),
+    NSToIntRound(dpiScale * inTop),
+    NSToIntRound(dpiScale * (inLeft + inWidth)),
+    NSToIntRound(dpiScale * (inTop + inHeight))
+  };
+
+  HMONITOR genScreen = ::MonitorFromRect( &globalWindowBounds, MONITOR_DEFAULTTOPRIMARY );
+
+  *outScreen = CreateNewScreenObject ( genScreen );    // addrefs
+  
+  return NS_OK;
+    
+} // ScreenForRect
+
+
+//
+// GetPrimaryScreen
+//
+// The screen with the menubar/taskbar. This shouldn't be needed very
+// often.
+//
+NS_IMETHODIMP 
+nsScreenManagerWin :: GetPrimaryScreen(nsIScreen** aPrimaryScreen) 
+{
+  *aPrimaryScreen = CreateNewScreenObject ( nullptr );    // addrefs  
+  return NS_OK;
+  
+} // GetPrimaryScreen
+
+
+//
+// CountMonitors
+//
+// Will be called once for every monitor in the system. Just 
+// increments the parameter, which holds a ptr to a PRUin32 holding the
+// count up to this point.
+//
+BOOL CALLBACK
+CountMonitors ( HMONITOR, HDC, LPRECT, LPARAM ioParam )
+{
+  uint32_t* countPtr = reinterpret_cast<uint32_t*>(ioParam);
+  ++(*countPtr);
+
+  return TRUE; // continue the enumeration
+
+} // CountMonitors
+
+
+//
+// GetNumberOfScreens
+//
+// Returns how many physical screens are available.
+//
+NS_IMETHODIMP
+nsScreenManagerWin :: GetNumberOfScreens(uint32_t *aNumberOfScreens)
+{
+  if ( mNumberOfScreens )
+    *aNumberOfScreens = mNumberOfScreens;
+  else {
+    uint32_t count = 0;
+    BOOL result = ::EnumDisplayMonitors(nullptr, nullptr, (MONITORENUMPROC)CountMonitors, (LPARAM)&count);
+    if (!result)
+      return NS_ERROR_FAILURE;
+    *aNumberOfScreens = mNumberOfScreens = count;
+  }
+
+  return NS_OK;
+  
+} // GetNumberOfScreens
+
+NS_IMETHODIMP
+nsScreenManagerWin::GetSystemDefaultScale(float *aDefaultScale)
+{
+  *aDefaultScale = float(gfxWindowsPlatform::GetPlatform()->GetDPIScale());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsScreenManagerWin :: ScreenForNativeWidget(void *aWidget, nsIScreen **outScreen)
+{
+  HMONITOR mon = MonitorFromWindow ((HWND) aWidget, MONITOR_DEFAULTTOPRIMARY);
+  *outScreen = CreateNewScreenObject (mon);
+  return NS_OK;
+}