Bug 929915. Fix IsScrollingActive on scroll frames to only be true if we actually build a scrollable layer. r=roc

layout/generic/nsGfxScrollFrame.cpp

@@ -1560,7 +1560,7 @@ nsGfxScrollFrameInner::nsGfxScrollFrameInner(nsContainerFrame* aOuter,
   , mMayHaveDirtyFixedChildren(false)
   , mUpdateScrollbarAttributes(false)
   , mCollapsedResizer(false)
-  , mShouldBuildLayer(false)
+  , mShouldBuildScrollableLayer(false)
   , mHasBeenScrolled(false)
 {
   mScrollingActive = IsAlwaysActive();
@@ -2131,12 +2131,6 @@ nsGfxScrollFrameInner::AppendScrollPartsTo(nsDisplayListBuilder*   aBuilder,
   }
 }
 
-bool
-nsGfxScrollFrameInner::ShouldBuildLayer() const
-{
-  return mShouldBuildLayer;
-}
-
 class ScrollLayerWrapper : public nsDisplayWrapper
 {
 public:
@@ -2343,8 +2337,9 @@ nsGfxScrollFrameInner::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   // When a displayport is being used, force building of a layer so that
   // CompositorParent can always find the scrollable layer for the root content
   // document.
+  bool shouldBuildLayer = false;
   if (usingDisplayport) {
-    mShouldBuildLayer = true;
+    shouldBuildLayer = true;
   } else {
     nsRect scrollRange = GetScrollRange();
     ScrollbarStyles styles = GetScrollbarStylesFromFrame();
@@ -2359,18 +2354,20 @@ nsGfxScrollFrameInner::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
       wantSubAPZC = true;
     }
 #endif
-    mShouldBuildLayer =
+    shouldBuildLayer =
       wantSubAPZC &&
       hasScrollableOverflow &&
       (!mIsRoot || !mOuter->PresContext()->IsRootContentDocument());
   }
 
-  if (ShouldBuildLayer()) {
+  mShouldBuildScrollableLayer = false;
+  if (shouldBuildLayer) {
     // ScrollLayerWrapper must always be created because it initializes the
     // scroll layer count. The display lists depend on this.
     ScrollLayerWrapper wrapper(mOuter, mScrolledFrame);
 
     if (usingDisplayport) {
+      mShouldBuildScrollableLayer = true;
       DisplayListClipState::AutoSaveRestore clipState(aBuilder);
 
       // For root scrollframes in documents where the CSS viewport has been

layout/generic/nsGfxScrollFrame.h

@@ -65,8 +65,6 @@ public:
   void PostOverflowEvent();
   void Destroy();
 
-  bool ShouldBuildLayer() const;
-
   void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                         const nsRect&           aDirtyRect,
                         const nsDisplayListSet& aLists);
@@ -268,7 +266,7 @@ public:
   nscoord GetNondisappearingScrollbarWidth(nsBoxLayoutState* aState);
   bool IsLTR() const;
   bool IsScrollbarOnRight() const;
-  bool IsScrollingActive() const { return mScrollingActive || ShouldBuildLayer(); }
+  bool IsScrollingActive() const { return mScrollingActive || mShouldBuildScrollableLayer; }
   void ResetScrollPositionForLayerPixelAlignment()
   {
     mScrollPosForLayerPixelAlignment = GetScrollPosition();
@@ -386,9 +384,9 @@ public:
   // If true, the resizer is collapsed and not displayed
   bool mCollapsedResizer:1;
 
-  // If true, the layer should always be active because we always build a layer.
-  // Used for asynchronous scrolling.
-  bool mShouldBuildLayer:1;
+  // If true, the layer should always be active because we always build a
+  // scrollable layer. Used for asynchronous scrolling.
+  bool mShouldBuildScrollableLayer:1;
 
   // True if this frame has been scrolled at least once
   bool mHasBeenScrolled:1;