Enable compositing for opaque scrolling content on low DPI screens

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 cb5500a93959ed72957af3adadf0b1c9d534d3bc..2303e74519c2fb0f0b51799e085a66128869b8b2 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -221,8 +221,7 @@ GraphicsLayer* PaintLayerScrollableArea::layerForScrollCorner() const
 
 bool PaintLayerScrollableArea::shouldUseIntegerScrollOffset() const
 {
-    Frame* frame = box().frame();
-    if (frame->settings() && !frame->settings()->preferCompositingToLCDTextEnabled())
+    if (m_needsCompositedScrolling)
         return true;
 
     return ScrollableArea::shouldUseIntegerScrollOffset();
@@ -1494,13 +1493,18 @@ static bool layerNeedsCompositedScrolling(PaintLayerScrollableArea::LCDTextMode
     if (node && node->isElementNode() && (toElement(node)->compositorMutableProperties() & (CompositorMutableProperty::kScrollTop | CompositorMutableProperty::kScrollLeft)))
         return true;
 
-    if (mode == PaintLayerScrollableArea::ConsiderLCDText && !layer->compositor()->preferCompositingToLCDTextEnabled())
+    if (mode == PaintLayerScrollableArea::ConsiderLCDText
+        && !layer->compositor()->preferCompositingToLCDTextEnabled()
+        && (!RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled()
+            || layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha()))
         return false;
 
-    return !layer->size().isEmpty()
-        && !layer->hasDescendantWithClipPath()
-        && !layer->hasAncestorWithClipPath()
-        && !layer->layoutObject()->style()->hasBorderRadius();
+    return !(layer->size().isEmpty()
+        || layer->hasDescendantWithClipPath()
+        || layer->hasAncestorWithClipPath()
+        || layer->layoutObject()->isFloating()
+        || layer->layoutObject()->style()->hasBorderRadius()
+        || layer->layoutObject()->style()->hasBorderDecoration());

[flackr, 2016/07/27 22:17:27]

Why can we not use composited scrolling with a border decoration or floating
elements? Won't this reduce the number of composited scrolling cases we have on
High DPI today?

[Stephen Chennney, 2016/07/28 13:55:38]

Dashed borders are broken in tests because we don't draw the background behind
the dashes (might be a test problem using the wrong box). Floating layers give
assertions about selfPaintingLayer.

These tests fail today with existing promotion on HighDPI devices, I believe,
but we don't really look at the results.

[flackr, 2016/07/28 14:24:25]

Understood, sounds good. Can we land these as a separate change with high dpi
tests that will fail if we don't have these extra conditions?

[Stephen Chennney, 2016/07/28 14:50:47]

I'm not sure what you mean. Do you mean leave out these extra conditions in the
code and address them in a separate change? If so, Chris also thought so and the
latest patch set does that. The follow up plan in that case is to fix the
underlying issues so that we don't need the extra rejection conditions.

[flackr, 2016/07/28 14:55:15]

Yes, basically, we can land these extra conditions ahead of this patch (or after
I suppose since we're not turning on promotion by default yet). Since it
reproduces on high dpi you should be able to demonstrate that these conditions
are necessary with a test in one of our VirtualTestSuites which enables high
dpi:
https://cs.chromium.org/chromium/src/third_party/WebKit/LayoutTests/VirtualTe...

 }
 
 void PaintLayerScrollableArea::updateNeedsCompositedScrolling(LCDTextMode mode)