--- a/layout/base/nsDisplayList.h
+++ b/layout/base/nsDisplayList.h
@@ -671,17 +671,36 @@ public:
   nsPresContext* CurrentPresContext() {
     return CurrentPresShellState()->mPresShell->GetPresContext();
   }
 
   /**
    * Accumulates the bounds of box frames that have moz-appearance
    * -moz-win-exclude-glass style. Used in setting glass margins on
    * Windows.
-   */  
+   *
+   * We set the window opaque region (from which glass margins are computed)
+   * to the intersection of the glass region specified here and the opaque
+   * region computed during painting. So the excluded glass region actually
+   * *limits* the extent of the opaque area reported to Windows. We limit it
+   * so that changes to the computed opaque region (which can vary based on
+   * region optimizations and the placement of UI elements) outside the
+   * -moz-win-exclude-glass area don't affect the glass margins reported to
+   * Windows; changing those margins willy-nilly can cause the Windows 7 glass
+   * haze effect to jump around disconcertingly.
+   */
+  void AddWindowExcludeGlassRegion(const nsRegion& bounds) {
+    mWindowExcludeGlassRegion.Or(mWindowExcludeGlassRegion, bounds);
+  }
+  const nsRegion& GetWindowExcludeGlassRegion() {
+    return mWindowExcludeGlassRegion;
+  }
+  /**
+   * Accumulates opaque stuff into the window opaque region.
+   */
   void AddWindowOpaqueRegion(const nsRegion& bounds) {
     mWindowOpaqueRegion.Or(mWindowOpaqueRegion, bounds);
   }
   /**
    * Returns the window opaque region built so far. This may be incomplete
    * since the opaque region is built during layer construction.
    */
   const nsRegion& GetWindowOpaqueRegion() {
@@ -750,16 +769,17 @@ private:
   // BuildDisplayList on.
   const nsIFrame*                mCurrentFrame;
   // The reference frame for mCurrentFrame.
   const nsIFrame*                mCurrentReferenceFrame;
   // The offset from mCurrentFrame to mCurrentReferenceFrame.
   nsPoint                        mCurrentOffsetToReferenceFrame;
   // Relative to mCurrentFrame.
   nsRect                         mDirtyRect;
+  nsRegion                       mWindowExcludeGlassRegion;
   nsRegion                       mWindowOpaqueRegion;
   nsRegion                       mWindowDraggingRegion;
   // The display item for the Windows window glass background, if any
   nsDisplayItem*                 mGlassDisplayItem;
   nsTArray<DisplayItemClip*>     mDisplayItemClipsToDestroy;
   Mode                           mMode;
   ViewID                         mCurrentScrollParentId;
   ViewID                         mCurrentScrollbarTarget;

--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -3132,21 +3132,22 @@ nsLayoutUtils::PaintFrame(nsRenderingCon
   // Update the widget's opaque region information. This sets
   // glass boundaries on Windows. Also set up the window dragging region
   // and plugin clip regions and bounds.
   if ((aFlags & PAINT_WIDGET_LAYERS) &&
       !willFlushRetainedLayers &&
       !(aFlags & PAINT_DOCUMENT_RELATIVE)) {
     nsIWidget *widget = aFrame->GetNearestWidget();
     if (widget) {
-      nsRegion opaqueRegion = builder.GetWindowOpaqueRegion();
+      nsRegion opaqueRegion;
+      opaqueRegion.And(builder.GetWindowExcludeGlassRegion(), builder.GetWindowOpaqueRegion());
       widget->UpdateOpaqueRegion(
         opaqueRegion.ToNearestPixels(presContext->AppUnitsPerDevPixel()));
 
-      nsRegion draggingRegion = builder.GetWindowDraggingRegion();
+      const nsRegion& draggingRegion = builder.GetWindowDraggingRegion();
       widget->UpdateWindowDraggingRegion(
         draggingRegion.ToNearestPixels(presContext->AppUnitsPerDevPixel()));
     }
   }
 
   if (builder.WillComputePluginGeometry()) {
     nsRefPtr<LayerManager> layerManager;
     nsIWidget* widget = aFrame->GetNearestWidget();

--- a/layout/xul/nsBoxFrame.cpp
+++ b/layout/xul/nsBoxFrame.cpp
@@ -1320,17 +1320,17 @@ nsBoxFrame::BuildDisplayList(nsDisplayLi
         forceLayer = (scrollTargetId != layers::FrameMetrics::NULL_SCROLL_ID);
         nsLayoutUtils::SetScrollbarThumbLayerization(this, forceLayer);
       }
     }
     // Check for frames that are marked as a part of the region used
     // in calculating glass margins on Windows.
     const nsStyleDisplay* styles = StyleDisplay();
     if (styles && styles->mAppearance == NS_THEME_WIN_EXCLUDE_GLASS) {
-      aBuilder->AddWindowOpaqueRegion(
+      aBuilder->AddWindowExcludeGlassRegion(
           nsRect(aBuilder->ToReferenceFrame(this), GetSize()));
     }
 
     aBuilder->AdjustWindowDraggingRegion(this);
   }
 
   nsDisplayListCollection tempLists;
   const nsDisplayListSet& destination = forceLayer ? tempLists : aLists;