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..9f44412cbe23d36e07160bef6283872b60cecfa2 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1494,13 +1494,24 @@ 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())
+    // 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()
+        && !layer->layoutObject()->style()->visitedDependentColor(CSSPropertyBackgroundColor).hasAlpha();
+    if (mode == PaintLayerScrollableArea::ConsiderLCDText
+        && !layer->compositor()->preferCompositingToLCDTextEnabled()
+        && !backgroundSupportsLCDText)
         return false;
 
-    return !layer->size().isEmpty()
-        && !layer->hasDescendantWithClipPath()
-        && !layer->hasAncestorWithClipPath()
-        && !layer->layoutObject()->style()->hasBorderRadius();
+    // TODO(schenney) Tests fail if we do not also exclude layer->layoutObject()->isFloating() (asserts) and

[chrishtr, 2016/08/04 16:35:44]

isFloating should have been fixed by https://codereview.chromium.org/2191953002.
The other is fixed by
better blacklisting above. Maybe just remove this TODO?

[Stephen Chennney, 2016/08/04 17:35:25]

Yep. Forgot you fixed that.

+    // layer->layoutObject()->style()->hasBorderDecoration() (missing background behind dashed borders).
+    // Resolve these cases, or not, and update this check with the results.
+    return !(layer->size().isEmpty()
+        || layer->hasDescendantWithClipPath()
+        || layer->hasAncestorWithClipPath()
+        || layer->layoutObject()->style()->hasBorderRadius());
 }
 
 void PaintLayerScrollableArea::updateNeedsCompositedScrolling(LCDTextMode mode)