Make composited scrolling codepaths co-operate.

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 116 matching lines @@
     : m_inResizeMode(false)
     , m_scrollDimensionsDirty(true)
     , m_normalFlowListDirty(true)
     , m_hasSelfPaintingLayerDescendant(false)
     , m_hasSelfPaintingLayerDescendantDirty(false)
     , m_hasOutOfFlowPositionedDescendant(false)
     , m_hasOutOfFlowPositionedDescendantDirty(true)
     , m_hasUnclippedDescendant(false)
     , m_isUnclippedDescendant(false)
     , m_needsCompositedScrolling(false)
+    , m_needsToBeStackingContainer(false)
     , m_canBePromotedToStackingContainer(false)
     , m_canBePromotedToStackingContainerDirty(true)
     , m_isRootLayer(renderer->isRenderView())
     , m_usedTransparency(false)
     , m_paintingInsideReflection(false)
     , m_inOverflowRelayout(false)
     , m_repaintStatus(NeedsNormalRepaint)
     , m_visibleContentStatusDirty(true)
     , m_hasVisibleContent(false)
     , m_visibleDescendantStatusDirty(false)
@@ skipping 323 matching lines @@
         }
     }
 }
 
 bool RenderLayer::acceleratedCompositingForOverflowScrollEnabled() const
 {
     const Settings* settings = renderer()->document()->settings();
     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
 {
     if (!acceleratedCompositingForOverflowScrollEnabled())
         return false;
 
     const Settings* settings = renderer()->document()->settings();
     return settings && settings->isCompositorDrivenAcceleratedScrollingEnabled();
 }
@@ skipping 1556 matching lines @@
 
     // Scroll form controls on the main thread so they exhibit correct touch scroll event bubbling
     if (box && (box->isIntristicallyScrollable(VerticalScrollbar) || box->isIntristicallyScrollable(HorizontalScrollbar)))
         return false;
 
     return isComposited() && backing()->scrollingLayer();
 }
 
 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;
     case CompositedScrollingAlwaysOn:
         return true;
     case CompositedScrollingAlwaysOff:
         return false;
     }
 
     ASSERT_NOT_REACHED();
     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");
 
     updateCanBeStackingContainer();
     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
     // number of composited overflow divs.
     if (acceleratedCompositingForOverflowScrollEnabled())
         HistogramSupport::histogramEnumeration("Renderer.NeedsCompositedScrolling", needsCompositedScrolling, 2);
 
     setNeedsCompositedScrolling(needsCompositedScrolling);
 }
 
@@ skipping 4419 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif