Record box shadow as main thread scrolling reasons

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>
@@ skipping 1734 matching lines @@
       (toElement(node)->compositorMutableProperties() &
        (CompositorMutableProperty::kScrollTop |
         CompositorMutableProperty::kScrollLeft)))
     return true;
 
   if (layer->size().isEmpty())
     return false;
 
   bool needsCompositedScrolling = true;
 
+  bool hasTranslucentBorder =
+      layer->layoutObject()
+          ->style()
+          ->visitedDependentColor(CSSPropertyBorderLeftColor)
+          .hasAlpha() ||
+      layer->layoutObject()
+          ->style()
+          ->visitedDependentColor(CSSPropertyBorderRightColor)
+          .hasAlpha() ||
+      layer->layoutObject()
+          ->style()
+          ->visitedDependentColor(CSSPropertyBorderTopColor)
+          .hasAlpha() ||
+      layer->layoutObject()
+          ->style()
+          ->visitedDependentColor(CSSPropertyBorderBottomColor)
+          .hasAlpha();
   // TODO(flackr): Allow integer transforms as long as all of the ancestor
   // transforms are also integer.
   bool backgroundSupportsLCDText =
       RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
       layer->layoutObject()->style()->isStackingContext() &&
       layer->backgroundPaintLocation() & BackgroundPaintInScrollingContents &&
       layer->backgroundIsKnownToBeOpaqueInRect(
           toLayoutBox(layer->layoutObject())->paddingBoxRect()) &&
-      !layer->compositesWithTransform() && !layer->compositesWithOpacity();
+      !layer->compositesWithTransform() && !layer->compositesWithOpacity() &&
+      !layer->layoutObject()->style()->boxShadow();

[bokan, 2017/01/17 21:36:55]

This looks like it's doing more than just recording, this change looks like
we're disabling compositing if the box has a shadow. If that's already the case,
where does that happen? Similarly with hasTranslucentBorder - how come we have
to add the condition above? Where does it actually disable composited scrolling?

[flackr, 2017/01/17 21:59:42]

We have this condition in
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/La...
which means the background cannot paint in the scrolling contents (line 1777
above). Seems that we should have a way of recording the reasons for the
background paint location so they can be added at the same place in the code
where they're detected. I think we can drop translucent borders now that the
general case is handled (we paint the BG in both the scrolling contents layer
and graphics layer).

[yigu, 2017/01/23 18:53:51]

Done.

 
   if (mode == PaintLayerScrollableArea::ConsiderLCDText &&
       !layer->compositor()->preferCompositingToLCDTextEnabled() &&
       !backgroundSupportsLCDText) {
     if (layer->compositesWithOpacity()) {
       addStyleRelatedMainThreadScrollingReasons(
           MainThreadScrollingReason::kHasOpacityAndLCDText);
     }
     if (layer->compositesWithTransform()) {
       addStyleRelatedMainThreadScrollingReasons(
           MainThreadScrollingReason::kHasTransformAndLCDText);
     }
     if (!layer->backgroundIsKnownToBeOpaqueInRect(
             toLayoutBox(layer->layoutObject())->paddingBoxRect())) {
       addStyleRelatedMainThreadScrollingReasons(
           MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText);
     }
+    if (layer->layoutBox()->style()->boxShadow()) {
+      addStyleRelatedMainThreadScrollingReasons(
+          MainThreadScrollingReason::kHasBoxShadowAndLCDText);
+    }
+    if (!RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
+        hasTranslucentBorder) {
+      addStyleRelatedMainThreadScrollingReasons(
+          MainThreadScrollingReason::kHasTranslucentBorderAndLCDText);
+    }
     needsCompositedScrolling = false;
   }
 
   // TODO(schenney) Tests fail if we do not also exclude
   // layer->layoutObject()->style()->hasBorderDecoration() (missing background
   // behind dashed borders). Resolve this case, or not, and update this check
   // with the results.
   if (layer->layoutObject()->style()->hasBorderRadius()) {
     addStyleRelatedMainThreadScrollingReasons(
         MainThreadScrollingReason::kHasBorderRadius);
@@ skipping 314 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink