Make composited scrolling codepaths co-operate.

Source/core/rendering/RenderLayer.cpp

Index: Source/core/rendering/RenderLayer.cpp
diff --git a/Source/core/rendering/RenderLayer.cpp b/Source/core/rendering/RenderLayer.cpp
index b164a9e885109df4fcce68dc42a2b3b3f3c272dc..6176e54e9295e840154c4771423a979db7ed0fc4 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -134,6 +134,7 @@ RenderLayer::RenderLayer(RenderLayerModelObject* renderer)
     , m_hasUnclippedDescendant(false)
     , m_isUnclippedDescendant(false)
     , m_needsCompositedScrolling(false)
+    , m_needsToBeStackingContainer(false)
     , m_canBePromotedToStackingContainer(false)
     , m_canBePromotedToStackingContainerDirty(true)
     , m_isRootLayer(renderer->isRenderView())
@@ -477,6 +478,26 @@ bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
     return settings && settings->acceleratedCompositingForOverflowScrollEnabled();
 }
 
+bool RenderLayer::useCompositorDrivenAcceleratedScrolling() const
+{
+    TRACE_EVENT0("comp-scroll", "RenderLayer::useCompositorDrivenAcceleratedScrolling");

[Ian Vollick, 2013/08/29 03:05:01]

Can you ditch the printf-y trace events for now? I've got another batch of
these, and they're pretty contentious, so maybe we could roll this in with them.

[hartmanng, 2013/09/05 20:43:58]

Done.

+
+    if (!compositorDrivenAcceleratedScrollingEnabled())
+        return false;
+
+    const RenderLayer* scrollingLayer = ancestorScrollingLayer();
+    if (!scrollingLayer || scrollingLayer->isStackingContainer())
+        return false;
+
+    if (scrollingLayer->m_canBePromotedToStackingContainerDirty) {
+        // We don't know if it's safe to promote to stacking container, and
+        // aren't in a position to find out, so we have to assume the worst.
+        return true;
+    }
+
+    return !scrollingLayer->canBeStackingContainer();
+}
+
 // FIXME: This is a temporary flag and should be removed once accelerated
 // overflow scroll is ready (crbug.com/254111).
 bool RenderLayer::compositorDrivenAcceleratedScrollingEnabled() const
@@ -2053,15 +2074,6 @@ bool RenderLayer::usesCompositedScrolling() const
 
 bool RenderLayer::needsCompositedScrolling() const
 {
-    if (!compositorDrivenAcceleratedScrollingEnabled())
-        return needsToBeStackingContainer();
-    if (FrameView* frameView = renderer()->view()->frameView())
-        return frameView->containsScrollableArea(scrollableArea());
-    return false;
-}
-
-bool RenderLayer::needsToBeStackingContainer() const
-{
     switch (m_forceNeedsCompositedScrolling) {
     case DoNotForceCompositedScrolling:
         return m_needsCompositedScrolling;
@@ -2075,6 +2087,21 @@ bool RenderLayer::needsToBeStackingContainer() const
     return m_needsCompositedScrolling;
 }
 
+bool RenderLayer::needsToBeStackingContainer() const
+{

[Ian Vollick, 2013/08/29 03:05:01]

Repeating the switch is a bummer. Maybe we could have a helper bool
RenderLayer::adjustForForceCompositedScrollingMode(bool value)?

[hartmanng, 2013/09/05 20:43:58]

Done.

+    switch (m_forceNeedsCompositedScrolling) {
+    case DoNotForceCompositedScrolling:
+        return m_needsToBeStackingContainer;
+    case CompositedScrollingAlwaysOn:
+        return true;
+    case CompositedScrollingAlwaysOff:
+        return false;
+    }
+
+    ASSERT_NOT_REACHED();
+    return m_needsToBeStackingContainer;
+}
+
 void RenderLayer::updateNeedsCompositedScrolling()
 {
     TRACE_EVENT0("comp-scroll", "RenderLayer::updateNeedsCompositedScrolling");
@@ -2083,10 +2110,16 @@ void RenderLayer::updateNeedsCompositedScrolling()
     updateDescendantDependentFlags();
 
     ASSERT(renderer()->view()->frameView() && renderer()->view()->frameView()->containsScrollableArea(scrollableArea()));
-    bool needsCompositedScrolling = acceleratedCompositingForOverflowScrollEnabled()
+    const bool needsToBeStackingContainer = acceleratedCompositingForOverflowScrollEnabled()
         && canBeStackingContainer()
         && !hasUnclippedDescendant();
 
+    setNeedsToBeStackingContainer(needsToBeStackingContainer);
+
+    const bool needsCompositedScrolling = needsToBeStackingContainer
+        || (compositorDrivenAcceleratedScrollingEnabled()
+            && renderer()->view()->frameView()->containsScrollableArea(scrollableArea()));

[Ian Vollick, 2013/08/29 03:05:01]

We've already asserted that
renderer()->view()->frameView()->containsScrollableArea(scrollableArea()))
above. Shouldn't be necessary here.

[hartmanng, 2013/09/05 20:43:58]

Done.

+
     // We gather a boolean value for use with Google UMA histograms to
     // quantify the actual effects of a set of patches attempting to
     // relax composited scrolling requirements, thereby increasing the