Fix Compositing of Opaque Scrolling Layers and Add Tests

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 9237e04590abe81fc6949bfd8b2280c75224019d..aef397414193e30fb1bd4ebb658d8e057de904e8 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1502,7 +1502,8 @@ static bool layerNeedsCompositedScrolling(PaintLayerScrollableArea::LCDTextMode
     // 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()
-        && !layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha();
+        && layer->shouldPaintBackgroundOntoScrollingContentsLayer()

[Stephen Chennney, 2016/08/26 18:01:41]

My understanding is that here we cannot use the CompositedLayerMapping flag
because it will not have been updated yet.

[flackr, 2016/08/26 18:24:23]

I believe that's correct - but should be easy to test.

Have you also verified that we have the up to date value for not having negative
z index children? See !m_owningLayer.stackingNode()->hasNegativeZOrderList() at
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/co...

[Stephen Chennney, 2016/08/26 18:42:08]

I'll check.

+        && layer->backgroundIsKnownToBeOpaqueInRect(toLayoutBox(layer->layoutObject())->paddingBoxRect());

[Stephen Chennney, 2016/08/26 20:46:01]

We have a chicken and egg problem. The first thing required in
backgroundIsKnownToBeOpaqueInRect is that the layer is self painting, but the
code to determine self painting status calls this method to check if we need
overflow scrolling, so we never set isSelfPaintingLayer.

So here we need something else that is not concerned about what the layer is
painting into but is only concerned with the background itself.

Interestingly, in the unit test this manages to work because the layer is self
painting, and I have no idea why yet.

     if (mode == PaintLayerScrollableArea::ConsiderLCDText
         && !layer->compositor()->preferCompositingToLCDTextEnabled()
         && !backgroundSupportsLCDText)