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()

[chrishtr, 2016/07/27 18:45:44]

Why the two new things listed here?

[Stephen Chennney, 2016/07/27 19:13:26]

Because Floating layers trigger a isSelfPaintingLayer assertion if composited,
and because dashed borders do not paint correctly if composited. That might be
related to the one seen in the background-paints-in-foreground patch but the set
of test failures is different. I think this case is due to shrinking the
background to not draw in the border area, but we need it to if the border is
dashed and the background is visible behind it.

[chrishtr, 2016/07/27 19:26:00]

Please remove these then. Let's address them in a followup. I'd like to debug
with you whether they will work (sounds like the second one might start working
with flackr's patch).

[Stephen Chennney, 2016/07/27 22:19:53]

Done.

+        || layer->layoutObject()->style()->hasBorderRadius()
+        || layer->layoutObject()->style()->hasBorderDecoration());
 }
 
 void PaintLayerScrollableArea::updateNeedsCompositedScrolling(LCDTextMode mode)