Bug 870917: Fix overlay scrollbars in XUL trees to fade and disappear correctly instead of pulsating continuously. r=roc

layout/xul/tree/nsTreeBodyFrame.cpp

@@ -114,6 +114,7 @@ nsTreeBodyFrame::nsTreeBodyFrame(nsIPresShell* aPresShell, nsStyleContext* aCont
  mTopRowIndex(0),
  mPageLength(0),
  mHorzPosition(0),
+ mOriginalHorzWidth(-1),
  mHorzWidth(0),
  mAdjustWidth(0),
  mRowHeight(0),
@@ -384,16 +385,29 @@ nsTreeBodyFrame::EnsureView()
   }
 }
 
+void
+nsTreeBodyFrame::ManageReflowCallback(const nsRect& aRect, nscoord aHorzWidth)
+{
+  if (!mReflowCallbackPosted &&
+      (!aRect.IsEqualEdges(mRect) || mHorzWidth != aHorzWidth)) {
+    PresContext()->PresShell()->PostReflowCallback(this);
+    mReflowCallbackPosted = true;
+    mOriginalHorzWidth = mHorzWidth;
+  }
+  else if (mReflowCallbackPosted &&
+           mHorzWidth != aHorzWidth && mOriginalHorzWidth == aHorzWidth) {
+    PresContext()->PresShell()->CancelReflowCallback(this);
+    mReflowCallbackPosted = false;
+    mOriginalHorzWidth = -1;
+  }
+}
+
 void
 nsTreeBodyFrame::SetBounds(nsBoxLayoutState& aBoxLayoutState, const nsRect& aRect,
                            bool aRemoveOverflowArea)
 {
   nscoord horzWidth = CalcHorzWidth(GetScrollParts());
-  if ((!aRect.IsEqualEdges(mRect) || mHorzWidth != horzWidth) && !mReflowCallbackPosted) {
-    mReflowCallbackPosted = true;
-    PresContext()->PresShell()->PostReflowCallback(this);
-  }
-
+  ManageReflowCallback(aRect, horzWidth);
   mHorzWidth = horzWidth;
 
   nsLeafBoxFrame::SetBounds(aBoxLayoutState, aRect, aRemoveOverflowArea);

layout/xul/tree/nsTreeBodyFrame.h

@@ -120,6 +120,8 @@ public:
   nsresult EndUpdateBatch();
   nsresult ClearStyleAndImageCaches();
 
+  void ManageReflowCallback(const nsRect& aRect, nscoord aHorzWidth);
+
   virtual nsSize GetMinSize(nsBoxLayoutState& aBoxLayoutState) MOZ_OVERRIDE;
   virtual void SetBounds(nsBoxLayoutState& aBoxLayoutState, const nsRect& aRect,
                          bool aRemoveOverflowArea = false) MOZ_OVERRIDE;
@@ -576,6 +578,13 @@ protected: // Data Members
 
   // The horizontal scroll position
   nscoord mHorzPosition;
+
+  // The original desired horizontal width before changing it and posting a
+  // reflow callback. In some cases, the desired horizontal width can first be
+  // different from the current desired horizontal width, only to return to
+  // the same value later during the same reflow. In this case, we can cancel
+  // the posted reflow callback and prevent an unnecessary reflow.
+  nscoord mOriginalHorzWidth;
   // Our desired horizontal width (the width for which we actually have tree
   // columns).
   nscoord mHorzWidth;