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

[chrishtr, 2016/09/02 01:19:38]

Are you checking for integer translation?

[Stephen Chennney, 2016/09/06 21:06:21]

I could DCHECK for it. I believe we force integer scrolling if
needsCompositedScrolling is true, so we're OK.

[chrishtr, 2016/09/06 22:17:59]

The sum of the translations from the root must be integers to get LCD text. I
think this needs more investigation. Please file a bug. and add a TODO here.

[Stephen Chennney, 2016/09/07 19:57:54]

This comment on an earlier review explains why we have integer scroll offset:
https://codereview.chromium.org/1826013002/#msg55

I've added the bug number and a comment, but not here, rather in
CompositingReasonsFinder where it makes more sense. Here we are just adding
CompositingReasonOverflowScrollingTouch for cases where we have already given up
on getting LCD Text. The forthcoming patch reworks this comment to reflect what
is actually happening.

-            // 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