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 218 matching lines @@
     if (currentRecursionData.m_hasCompositedScrollingAncestor && layer->layoutObject()->styleRef().hasViewportConstrainedPosition())
         directReasons |= CompositingReasonPositionFixed;
 
     bool canBeComposited = compositor->canBeComposited(layer);
     if (canBeComposited) {
         reasonsToComposite |= directReasons;
 
         if (layer->isRootLayer() && compositor->rootShouldAlwaysComposite())
             reasonsToComposite |= CompositingReasonRoot;
 
+        // Add CompositingReasonOverflowScrollingTouch for layers that do not already have it but could.

[chrishtr, 2016/09/09 20:32:14]

s/could/need it/

[Stephen Chennney, 2016/09/09 20:56:11]

Done.

+        // Note that m_compositingReasonFinder.directReasons(layer) already includes CompositingReasonOverflowScrollingTouch for
+        // anything that has layer->needsCompositedScrolling() true. That is, for cases where we explicitly decide not to have LCD
+        // text or cases where the layer will still support LCD text even if the layer is composited.
         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
-            // 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.
+            // We can get here for a scroller that will be composited for some other reason and hence will already
+            // use grayscale AA text. We recheck for needsCompositedScrolling ignoring LCD to correctly add the
+            // CompositingReasonOverflowScrollingTouch reason to layers that can support it with grayscale AA text.
             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 15 matching lines @@
             }
             if (layer->scrollsWithRespectTo(unclippedDescendant))
                 reasonsToComposite |= CompositingReasonAssumedOverlap;
         }
 
         // Remove irrelevant unclipped descendants in reverse order so our stored
         // indices remain valid.
         for (size_t i = 0; i < unclippedDescendantsToRemove.size(); i++)
             unclippedDescendants.remove(unclippedDescendantsToRemove.at(unclippedDescendantsToRemove.size() - i - 1));
 
-        if (reasonsToComposite & CompositingReasonOutOfFlowClipping)
+        if (layer->clipParent()) {
+            // TODO(schenney): We only need to promote when the clipParent is not a descendant of the ancestor scroller,
+            // which we do not check for here. Hence we might be promoting needlessly.
             unclippedDescendants.append(layer);
+        }
     }
 
     const IntRect& absBounds = layer->clippedAbsoluteBoundingBox();
     absoluteDescendantBoundingBox = absBounds;
 
     if (currentRecursionData.m_testingOverlap && !requiresCompositingOrSquashing(directReasons))
         overlapCompositingReason = overlapMap.overlapsLayers(absBounds) ? CompositingReasonOverlap : CompositingReasonNone;
 
     reasonsToComposite |= overlapCompositingReason;
 
@@ skipping 150 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