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 ae12437c9cdef7f295937445478a217fd6ffad36..1ea81a6c0a4499ba5d2b13adeccdcb2faf6567e4 100644
--- a/Source/core/rendering/RenderLayer.cpp
+++ b/Source/core/rendering/RenderLayer.cpp
@@ -495,6 +495,26 @@ bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
     return settings && settings->acceleratedCompositingForOverflowScrollEnabled();
 }
 
+bool RenderLayer::useCompositorDrivenAcceleratedScrolling() const
+{
+    TRACE_EVENT0("comp-scroll", "RenderLayer::useCompositorDrivenAcceleratedScrolling");
+
+    if (!compositorDrivenAcceleratedScrollingEnabled())
+        return false;
+
+    const RenderLayer* scrollingLayer = ancestorScrollingLayer();
+    if (!scrollingLayer || scrollingLayer->isStackingContainer())
+        return false;
+
+    if (scrollingLayer->m_canBePromotedToStackingContainerDirty) {

[Ian Vollick, 2013/08/08 03:30:10]

Can you describe how we get into the situation where we're asking if we're using
compositor driven accelerated scrolling when can-be-promoted.. is dirty?

[hartmanng, 2013/08/08 13:41:37]

The situation I ran into looks like this:

 [0x7f795c414a78]
WebCore::RenderLayer::useCompositorDrivenAcceleratedScrolling()
 [0x7f795c452179]
WebCore::RenderLayerCompositor::requiresCompositingForOverflowScrollingParent()
 [0x7f795c44db86] WebCore::RenderLayerCompositor::directReasonsForCompositing()
 [0x7f795c44e8dd] WebCore::RenderLayerCompositor::needsToBeComposited()
 [0x7f795c44e096] WebCore::RenderLayerCompositor::updateBacking()
 [0x7f795c44eba3] WebCore::RenderLayerCompositor::updateLayerCompositingState()
 [0x7f795c420f9e] WebCore::RenderLayer::updateScrollInfoAfterLayout()
 [0x7f795c2fe5d4] WebCore::RenderBlock::updateScrollInfoAfterLayout()
 [0x7f795c2ffc15] WebCore::RenderBlock::layoutBlock()
 [0x7f795c2fe65d] WebCore::RenderBlock::layout()


I considered doing a RenderLayer::updateCanBeStackingContainer() in
RenderLayerCompositor before we call
RenderLayer::useCompositorDrivenAcceleratedScrolling(), but unfortunately all
the RenderLayer instances around there are const, and
RenderLayer::updateCanBeStackingContainer() isn't.

+        // 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