Bug 1367488 - Pre-render offscreen portions of scrollbar thumbs inside an iframe. r=mstange

MozReview-Commit-ID: LCBHnFJdGtp

--HG--
extra : rebase_source : 1ab1d812674d321d76e634953284b3983774d798

layout/painting/FrameLayerBuilder.cpp

@@ -3935,6 +3935,13 @@ ContainerState::SetupMaskLayerForCSSMask(Layer* aLayer,
   aLayer->SetMaskLayer(maskLayer);
 }
 
+static bool
+IsScrollThumbLayer(nsDisplayItem* aItem)
+{
+  return aItem->GetType() == nsDisplayItem::TYPE_OWN_LAYER &&
+         static_cast<nsDisplayOwnLayer*>(aItem)->IsScrollThumbLayer();
+}
+
 /*
  * Iterate through the non-clip items in aList and its descendants.
  * For each item we compute the effective clip rect. Each item is assigned
@@ -4204,8 +4211,9 @@ ContainerState::ProcessDisplayItems(nsDisplayList* aList)
         mPaintedLayerDataTree.AddingOwnLayer(clipAGR,
                                              &scrolledClipRect,
                                              uniformColorPtr);
-      } else if (*animatedGeometryRoot == item->Frame() &&
-                 *animatedGeometryRoot != mBuilder->RootReferenceFrame()) {
+      } else if ((*animatedGeometryRoot == item->Frame() &&
+                  *animatedGeometryRoot != mBuilder->RootReferenceFrame()) ||
+                 (IsScrollThumbLayer(item) && mManager->IsWidgetLayerManager())) {
         // This is the case for scrollbar thumbs, for example. In that case the
         // clip we care about is the overflow:hidden clip on the scrollbar.
         mPaintedLayerDataTree.AddingOwnLayer(animatedGeometryRoot->mParentAGR,

layout/painting/nsDisplayList.cpp

@@ -6110,6 +6110,14 @@ nsDisplayOwnLayer::nsDisplayOwnLayer(nsDisplayListBuilder* aBuilder,
     , mForceActive(aForceActive)
 {
   MOZ_COUNT_CTOR(nsDisplayOwnLayer);
+
+  // For scroll thumb layers, override the AGR to be the thumb's AGR rather
+  // than the AGR for mFrame (which is the slider frame).
+  if (IsScrollThumbLayer()) {
+    if (nsIFrame* thumbFrame = nsBox::GetChildXULBox(mFrame)) {
+      mAnimatedGeometryRoot = aBuilder->FindAnimatedGeometryRootFor(thumbFrame);
+    }
+  }
 }
 
 #ifdef NS_BUILD_REFCNT_LOGGING
@@ -6130,6 +6138,20 @@ nsDisplayOwnLayer::GetLayerState(nsDisplayListBuilder* aBuilder,
   return RequiredLayerStateForChildren(aBuilder, aManager, aParameters, mList, mAnimatedGeometryRoot);
 }
 
+bool
+nsDisplayOwnLayer::IsScrollThumbLayer() const
+{
+  return (mFlags & VERTICAL_SCROLLBAR) || (mFlags & HORIZONTAL_SCROLLBAR);
+}
+
+bool
+nsDisplayOwnLayer::ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder)
+{
+  // Render scroll thumb layers even if they are invisible, because async
+  // scrolling might bring them into view.
+  return IsScrollThumbLayer();
+}
+
 // nsDisplayOpacity uses layers for rendering
 already_AddRefed<Layer>
 nsDisplayOwnLayer::BuildLayer(nsDisplayListBuilder* aBuilder,

layout/painting/nsDisplayList.h

@@ -1428,6 +1428,7 @@ private:
   AnimatedGeometryRoot* FindAnimatedGeometryRootFor(nsDisplayItem* aItem);
 
   friend class nsDisplayItem;
+  friend class nsDisplayOwnLayer;
   AnimatedGeometryRoot* FindAnimatedGeometryRootFor(nsIFrame* aFrame);
 
   AnimatedGeometryRoot* WrapAGRForFrame(nsIFrame* aAnimatedGeometryRoot,
@@ -4079,7 +4080,7 @@ public:
 #ifdef NS_BUILD_REFCNT_LOGGING
   virtual ~nsDisplayOwnLayer();
 #endif
-
+  virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder) override;
   virtual already_AddRefed<Layer> BuildLayer(nsDisplayListBuilder* aBuilder,
                                              LayerManager* aManager,
                                              const ContainerLayerParameters& aContainerParameters) override;
@@ -4095,6 +4096,7 @@ public:
     return false;
   }
   uint32_t GetFlags() { return mFlags; }
+  bool IsScrollThumbLayer() const;
   NS_DISPLAY_DECL_NAME("OwnLayer", TYPE_OWN_LAYER)
 protected:
   uint32_t mFlags;

layout/xul/nsSliderFrame.cpp

@@ -421,6 +421,22 @@ nsSliderFrame::BuildDisplayListForChildren(nsDisplayListBuilder*   aBuilder,
       nsRect dirty = aDirtyRect.Intersect(thumbRect);
       dirty = nsLayoutUtils::ComputePartialPrerenderArea(aDirtyRect, overflow, refSize);
 
+      // Clip the thumb layer to the slider track. This is necessary to ensure
+      // FrameLayerBuilder is able to merge content before and after the
+      // scrollframe into the same layer (otherwise it thinks the thumb could
+      // potentially move anywhere within the existing clip).
+      DisplayListClipState::AutoSaveRestore thumbClipState(aBuilder);
+      aBuilder->GetCurrentReferenceFrame();
+      thumbClipState.ClipContainingBlockDescendants(
+          GetRectRelativeToSelf() + aBuilder->ToReferenceFrame(this));
+
+      // Have the thumb's container layer capture the current clip, so
+      // it doesn't apply to the thumb's contents. This allows the contents
+      // to be fully rendered even if they're partially or fully offscreen,
+      // so async scrolling can still bring it into view.
+      DisplayListClipState::AutoSaveRestore thumbContentsClipState(aBuilder);
+      thumbContentsClipState.Clear();
+
       nsDisplayListBuilder::AutoContainerASRTracker contASRTracker(aBuilder);
       nsDisplayListCollection tempLists;
       nsBoxFrame::BuildDisplayListForChildren(aBuilder, dirty, tempLists);
@@ -435,10 +451,11 @@ nsSliderFrame::BuildDisplayListForChildren(nsDisplayListBuilder*   aBuilder,
       masterList.AppendToTop(tempLists.PositionedDescendants());
       masterList.AppendToTop(tempLists.Outlines());
 
+      // Restore the saved clip so it applies to the thumb container layer.
+      thumbContentsClipState.Restore();
+
       // Wrap the list to make it its own layer.
       const ActiveScrolledRoot* ownLayerASR = contASRTracker.GetContainerASR();
-      DisplayListClipState::AutoSaveRestore ownLayerClipState(aBuilder);
-      ownLayerClipState.ClearUpToASR(ownLayerASR);
       aLists.Content()->AppendNewToTop(new (aBuilder)
         nsDisplayOwnLayer(aBuilder, this, &masterList, ownLayerASR,
                           flags, scrollTargetId,