Enable compositing for opaque scrolling content on low DPI screens

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 203 matching lines @@
 GraphicsLayer* PaintLayerScrollableArea::layerForScrollCorner() const
 {
     // See crbug.com/343132.
     DisableCompositingQueryAsserts disabler;
 
     return layer()->hasCompositedLayerMapping() ? layer()->compositedLayerMapping()->layerForScrollCorner() : 0;
 }
 
 bool PaintLayerScrollableArea::shouldUseIntegerScrollOffset() const
 {
-    Frame* frame = box().frame();
-    if (frame->settings() && !frame->settings()->preferCompositingToLCDTextEnabled())
+    if (m_needsCompositedScrolling)
         return true;
 
     return ScrollableArea::shouldUseIntegerScrollOffset();
 }
 
 bool PaintLayerScrollableArea::isActive() const
 {
     Page* page = box().frame()->page();
     return page && page->focusController().isActive();
 }
@@ skipping 1251 matching lines @@
 
 static bool layerNeedsCompositedScrolling(PaintLayerScrollableArea::LCDTextMode mode, const PaintLayer* layer)
 {
     if (!layer->scrollsOverflow())
         return false;
 
     Node* node = layer->enclosingNode();
     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)
 {
     const bool needsCompositedScrolling = layerNeedsCompositedScrolling(mode, layer());
     if (static_cast<bool>(m_needsCompositedScrolling) != needsCompositedScrolling) {
         m_needsCompositedScrolling = needsCompositedScrolling;
         layer()->didUpdateNeedsCompositedScrolling();
     }
 }
@@ skipping 244 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink