Bug 642246 Don't build ThebesLayers for elements with no displayports r=roc

gfx/layers/Layers.h

@@ -762,6 +762,12 @@ public:
    */
   virtual ThebesLayer* AsThebesLayer() { return nsnull; }
 
+  /**
+   * Dynamic cast to a ContainerLayer. Returns null if this is not
+   * a ContainerLayer.
+   */
+  virtual ContainerLayer* AsContainerLayer() { return nsnull; }
+
   /**
    * Dynamic cast to a ShadowLayer.  Return null if this is not a
    * ShadowLayer.  Can be used anytime.
@@ -1029,6 +1035,8 @@ public:
 
   // These getters can be used anytime.
 
+  virtual ContainerLayer* AsContainerLayer() { return this; }
+
   virtual Layer* GetFirstChild() { return mFirstChild; }
   virtual Layer* GetLastChild() { return mLastChild; }
   const FrameMetrics& GetFrameMetrics() { return mFrameMetrics; }

layout/base/FrameLayerBuilder.h

@@ -59,7 +59,9 @@ enum LayerState {
   LAYER_ACTIVE,
   // Force an active layer even if it causes incorrect rendering, e.g.
   // when the layer has rounded rect clips.
-  LAYER_ACTIVE_FORCE
+  LAYER_ACTIVE_FORCE,
+  // Special layer that is metadata only.
+  LAYER_ACTIVE_EMPTY
 };
 
 /**

layout/base/nsDisplayItemTypes.h

@@ -84,6 +84,7 @@ enum Type {
   TYPE_REMOTE,
   TYPE_REMOTE_SHADOW,
   TYPE_SCROLL_LAYER,
+  TYPE_SCROLL_INFO_LAYER,
   TYPE_SELECTION_OVERLAY,
   TYPE_SOLID_COLOR,
   TYPE_TABLE_CELL_BACKGROUND,

layout/base/nsDisplayList.cpp

@@ -1836,6 +1836,25 @@ nsDisplayScrollLayer::~nsDisplayScrollLayer()
 }
 #endif
 
+nsDisplayScrollInfoLayer::nsDisplayScrollInfoLayer(
+  nsDisplayListBuilder* aBuilder,
+  nsDisplayList* aList,
+  nsIFrame* aForFrame,
+  nsIFrame* aViewportFrame)
+  : nsDisplayScrollLayer(aBuilder, aList, aForFrame, aViewportFrame)
+{
+#ifdef NS_BUILD_REFCNT_LOGGING
+  MOZ_COUNT_CTOR(nsDisplayScrollInfoLayer);
+#endif
+}
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+nsDisplayScrollInfoLayer::~nsDisplayScrollInfoLayer()
+{
+  MOZ_COUNT_DTOR(nsDisplayScrollInfoLayer);
+}
+#endif
+
 nsDisplayClip::nsDisplayClip(nsDisplayListBuilder* aBuilder,
                              nsIFrame* aFrame, nsDisplayItem* aItem,
                              const nsRect& aRect)

layout/base/nsDisplayList.h

@@ -1836,6 +1836,30 @@ private:
   nsIFrame* mViewportFrame;
 };
 
+/**
+ * Like nsDisplayScrollLayer, but only has metadata on the scroll frame. This
+ * creates a layer that has no Thebes child layer, but still allows the
+ * compositor process to know of the scroll frame's existence.
+ */
+class nsDisplayScrollInfoLayer : public nsDisplayScrollLayer
+{
+public:
+  nsDisplayScrollInfoLayer(nsDisplayListBuilder* aBuilder, nsDisplayList* aList,
+                           nsIFrame* aForFrame, nsIFrame* aViewportFrame);
+  NS_DISPLAY_DECL_NAME("ScrollInfoLayer", TYPE_SCROLL_INFO_LAYER)
+
+#ifdef NS_BUILD_REFCNT_LOGGING
+  virtual ~nsDisplayScrollInfoLayer();
+#endif
+
+  virtual LayerState GetLayerState(nsDisplayListBuilder* aBuilder,
+                                   LayerManager* aManager)
+  {
+    return mozilla::LAYER_ACTIVE_EMPTY;
+  }
+
+};
+
 /**
  * nsDisplayClip can clip a list of items, but we take a single item
  * initially and then later merge other items into it when we merge

layout/generic/nsGfxScrollFrame.cpp

@@ -1961,6 +1961,7 @@ nsGfxScrollFrameInner::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   dirtyRect.IntersectRect(aDirtyRect, mScrollPort);
 
   // Override the dirty rectangle if the displayport has been set.
+  PRBool usingDisplayport =
     nsLayoutUtils::GetDisplayPort(mOuter->GetContent(), &dirtyRect);
 
   nsDisplayListCollection set;
@@ -1976,30 +1977,42 @@ nsGfxScrollFrameInner::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   // range of 20 pixels to eliminate many gfx scroll frames from becoming a
   // layer.
   //
-  PRInt32 appUnitsPerDevPixel = presContext->AppUnitsPerDevPixel();
   nsRect scrollRange = GetScrollRange();
   ScrollbarStyles styles = GetScrollbarStylesFromFrame();
   mShouldBuildLayer =
      (XRE_GetProcessType() == GeckoProcessType_Content &&
      (styles.mHorizontal != NS_STYLE_OVERFLOW_HIDDEN ||
       styles.mVertical != NS_STYLE_OVERFLOW_HIDDEN) &&
-     (scrollRange.width >= NSIntPixelsToAppUnits(20, appUnitsPerDevPixel) ||
-      scrollRange.height >= NSIntPixelsToAppUnits(20, appUnitsPerDevPixel))) &&
-     (!mIsRoot || !mOuter->PresContext()->IsRootContentDocument());
+     (!mIsRoot || !mOuter->PresContext()->IsRootContentDocument()));
 
   if (ShouldBuildLayer()) {
+    nsDisplayList list;
+    if (usingDisplayport) {
+      // Once a displayport is set, assume that scrolling needs to be fast
+      // so create a layer with all the content inside. The compositor
+      // process will be able to scroll the content asynchronously.
+      //
       // Note that using StackingContext breaks z order, so the resulting
       // rendering can be incorrect for weird edge cases!
 
-    nsDisplayList list;
       rv = mScrolledFrame->BuildDisplayListForStackingContext(
         aBuilder, dirtyRect + mOuter->GetOffsetTo(mScrolledFrame), &list);
 
       nsDisplayScrollLayer* layerItem = new (aBuilder) nsDisplayScrollLayer(
         aBuilder, &list, mScrolledFrame, mOuter);
       set.Content()->AppendNewToTop(layerItem);
-  } else
-  {
+    } else {
+      // If there is no displayport set, there is no reason here to force a
+      // layer that needs a memory-expensive allocation, but the compositor
+      // process would still like to know that it exists.
+
+      nsDisplayScrollLayer* layerItem = new (aBuilder) nsDisplayScrollInfoLayer(
+        aBuilder, &list, mScrolledFrame, mOuter);
+      set.Content()->AppendNewToTop(layerItem);
+
+      rv = mOuter->BuildDisplayListForChild(aBuilder, mScrolledFrame, dirtyRect, set);
+    }
+  } else {
     rv = mOuter->BuildDisplayListForChild(aBuilder, mScrolledFrame, dirtyRect, set);
   }
 

layout/ipc/RenderFrameParent.cpp

@@ -139,13 +139,8 @@ FindViewForId(const ViewMap& aMap, ViewID aId)
 static const FrameMetrics*
 GetFrameMetrics(Layer* aLayer)
 {
-  // Children are not container layers, so they don't have frame metrics. Give
-  // them a blank metric.
-  if (!aLayer->GetFirstChild())
-    return NULL;
-
-  ContainerLayer* container = static_cast<ContainerLayer*>(aLayer);
-  return &container->GetFrameMetrics();
+  ContainerLayer* container = aLayer->AsContainerLayer();
+  return container ? &container->GetFrameMetrics() : NULL;
 }
 
 static nsIntPoint
@@ -346,10 +341,9 @@ BuildViewMap(ViewMap& oldContentViews, ViewMap& newContentViews,
              nsFrameLoader* aFrameLoader, Layer* aLayer,
              float aXScale = 1, float aYScale = 1)
 {
-  if (!aLayer->GetFirstChild())
+  ContainerLayer* container = aLayer->AsContainerLayer();
+  if (!container)
     return;
-
-  ContainerLayer* container = static_cast<ContainerLayer*>(aLayer);
   const FrameMetrics metrics = container->GetFrameMetrics();
   const ViewID scrollId = metrics.mScrollId;