Record box shadow as main thread scrolling reasons

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 d2c84bbc433e9098dddcd9f950068771ade68037..dc274855b0d94c2d0be7e4885dff2831e36fe2d9 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1752,6 +1752,23 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
 
   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 =
@@ -1760,7 +1777,8 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
       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() &&
@@ -1778,6 +1796,15 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
       addStyleRelatedMainThreadScrollingReasons(
           MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText);
     }
+    if (layer->layoutBox()->style()->boxShadow()) {
+      addStyleRelatedMainThreadScrollingReasons(
+          MainThreadScrollingReason::kHasBoxShadowAndLCDText);
+    }
+    if (!RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
+        hasTranslucentBorder) {
+      addStyleRelatedMainThreadScrollingReasons(
+          MainThreadScrollingReason::kHasTranslucentBorderAndLCDText);
+    }
     needsCompositedScrolling = false;
   }