Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.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@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 1482 matching lines @@
     if (!layer->scrollsOverflow())
         return false;
 
     Node* node = layer->enclosingNode();
     if (node && node->isElementNode() && (toElement(node)->compositorMutableProperties() & (CompositorMutableProperty::kScrollTop | CompositorMutableProperty::kScrollLeft)))
         return true;
 
     // TODO(schenney) The color test alone is inadequate. When https://codereview.chromium.org/2196583002 lands
     // we should use PaintLayer::shouldPaintBackgroundOntoForeground() because we will not still get
     // LCD text unless the conditions there are met. It also unifies logic for scrolling compositing decisions.
     bool backgroundSupportsLCDText = RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled()

[flackr, 2016/08/19 17:36:53]

Can we remove the flag now?

-        && !layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha();
+        && layer->shouldPaintBackgroundOntoScrollingContentsLayer()
+        && layer->backgroundIsKnownToBeOpaqueInRect(toLayoutBox(layer->layoutObject())->paddingBoxRect());
     if (mode == PaintLayerScrollableArea::ConsiderLCDText
         && !layer->compositor()->preferCompositingToLCDTextEnabled()
         && !backgroundSupportsLCDText)
         return false;
 
     // TODO(schenney) Tests fail if we do not also exclude layer->layoutObject()->style()->hasBorderDecoration()
     // (missing background behind dashed borders). Resolve this case, or not, and update this check with
     // the results.
     return !(layer->size().isEmpty()
         || layer->hasDescendantWithClipPath()
@@ skipping 256 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink