Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/layout/compositing/CompositingRequirementsUpdater.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 220 matching lines @@
 
     bool canBeComposited = compositor->canBeComposited(layer);
     if (canBeComposited) {
         reasonsToComposite |= directReasons;
 
         if (layer->isRootLayer() && compositor->rootShouldAlwaysComposite())
             reasonsToComposite |= CompositingReasonRoot;
 
         if (reasonsToComposite && layer->scrollsOverflow() && !layer->needsCompositedScrolling()) {
             // We will only set needsCompositedScrolling if we don't care about
-            // the LCD text hit, we may be able to switch to the compositor
+            // the LCD text hit, or when the background supports painting LCD text.
+            // We may be able to switch to the compositor
             // driven path if we're alread composited for other reasons and are
             // therefore using grayscale AA.
-            //
-            // FIXME: it should also be possible to promote if the layer can
-            // still use LCD text when promoted, but detecting when the
-            // compositor can do this is tricky. Currently, the layer must be
-            // both opaque and may only have an integer translation as its
-            // transform. Both opacity and screen space transform are inherited
-            // properties, so this cannot be determined from local information.
             layer->getScrollableArea()->updateNeedsCompositedScrolling(PaintLayerScrollableArea::IgnoreLCDText);
             if (layer->needsCompositedScrolling())
                 reasonsToComposite |= CompositingReasonOverflowScrollingTouch;
         }
     }
 
     if ((reasonsToComposite & CompositingReasonOverflowScrollingTouch) && !layer->isRootLayer())
         currentRecursionData.m_hasCompositedScrollingAncestor = true;
 
     // Next, accumulate reasons related to overlap.
@@ skipping 187 matching lines @@
             overlapMap.finishCurrentOverlapTestingContext();
 
         descendantHas3DTransform |= anyDescendantHas3DTransform || layer->has3DTransform();
     }
 
     // At this point we have finished collecting all reasons to composite this layer.
     layer->setCompositingReasons(reasonsToComposite);
 }
 
 } // namespace blink