Make 'will-change: contents' suppress compositing in subtree

Source/core/rendering/compositing/CompositingReasonFinder.cpp

Index: Source/core/rendering/compositing/CompositingReasonFinder.cpp
diff --git a/Source/core/rendering/compositing/CompositingReasonFinder.cpp b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
index 37a3458487cb4d6337cce40626f43963d4ef6c0e..4eeca79afea82c6e1f5012f1bcd92344da58e867 100644
--- a/Source/core/rendering/compositing/CompositingReasonFinder.cpp
+++ b/Source/core/rendering/compositing/CompositingReasonFinder.cpp
@@ -110,10 +110,10 @@ CompositingReasons CompositingReasonFinder::styleDeterminedReasons(RenderObject*
     if (style->backfaceVisibility() == BackfaceVisibilityHidden)
         directReasons |= CompositingReasonBackfaceVisibilityHidden;
 
-    if (style->shouldCompositeForCurrentAnimations())
+    if (requiresCompositingForAnimation(renderer))

[abarth-chromium, 2014/06/06 17:18:29]

Why not pass in the RenderStyle given that these are style-determined reasons?

[ajuma, 2014/06/06 17:50:09]

Good point. CL up to fix this: https://codereview.chromium.org/317013005/

         directReasons |= CompositingReasonActiveAnimation;
 
-    if (style->hasWillChangeCompositingHint())
+    if (style->hasWillChangeCompositingHint() && !style->subtreeWillChangeContents())
         directReasons |= CompositingReasonWillChangeCompositingHint;
 
     ASSERT(!(directReasons & ~CompositingReasonComboAllStyleDeterminedReasons));
@@ -166,6 +166,14 @@ CompositingReasons CompositingReasonFinder::nonStyleDeterminedDirectReasons(cons
     return directReasons;
 }
 
+bool CompositingReasonFinder::requiresCompositingForAnimation(RenderObject* renderer) const
+{
+    if (renderer->style()->subtreeWillChangeContents())
+        return renderer->style()->isRunningAnimationOnCompositor();
+
+    return renderer->style()->shouldCompositeForCurrentAnimations();
+}
+
 bool CompositingReasonFinder::requiresCompositingForPosition(RenderObject* renderer, const RenderLayer* layer, RenderLayer::ViewportConstrainedNotCompositedReason* viewportConstrainedNotCompositedReason) const
 {
     return requiresCompositingForPositionSticky(renderer, layer) || requiresCompositingForPositionFixed(renderer, layer, viewportConstrainedNotCompositedReason);