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 58f4ef34c8fafea1ed661c7ab4a9986c04be3bba..869ac38ec914728c5fab0d2e3f4efee664f6f11e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1756,6 +1756,9 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
     return false;
 
   bool needsCompositedScrolling = true;
+  uint32_t mainThreadScrollingReasons = 0;
+  uint32_t* mainThreadScrollingReasonsDueToBackgroundPaintLocation =

[bokan, 2017/01/23 19:55:51]

Why keep this separate variable? Just pass mainThreadScrollingReasons to
backgroundPaintLocation.

[yigu, 2017/01/23 20:21:23]

Done.

+      &mainThreadScrollingReasons;
 
   // TODO(flackr): Allow integer transforms as long as all of the ancestor
   // transforms are also integer.
@@ -1771,18 +1774,23 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
       !layer->compositor()->preferCompositingToLCDTextEnabled() &&
       !backgroundSupportsLCDText) {
     if (layer->compositesWithOpacity()) {
-      addStyleRelatedMainThreadScrollingReasons(
-          MainThreadScrollingReason::kHasOpacityAndLCDText);
+      mainThreadScrollingReasons |=
+          MainThreadScrollingReason::kHasOpacityAndLCDText;
     }
     if (layer->compositesWithTransform()) {
-      addStyleRelatedMainThreadScrollingReasons(
-          MainThreadScrollingReason::kHasTransformAndLCDText);
+      mainThreadScrollingReasons |=
+          MainThreadScrollingReason::kHasTransformAndLCDText;
     }
     if (!layer->backgroundIsKnownToBeOpaqueInRect(
             toLayoutBox(layer->layoutObject())->paddingBoxRect())) {
-      addStyleRelatedMainThreadScrollingReasons(
-          MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText);
+      mainThreadScrollingReasons |=
+          MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText;
     }
+    if (layer->backgroundPaintLocation() & BackgroundPaintInGraphicsLayer) {
+      layer->layoutObject()->backgroundPaintLocation(
+          mainThreadScrollingReasonsDueToBackgroundPaintLocation);
+    }
+
     needsCompositedScrolling = false;
   }
 
@@ -1791,16 +1799,27 @@ bool PaintLayerScrollableArea::computeNeedsCompositedScrolling(
   // behind dashed borders). Resolve this case, or not, and update this check
   // with the results.
   if (layer->layoutObject()->style()->hasBorderRadius()) {
-    addStyleRelatedMainThreadScrollingReasons(
-        MainThreadScrollingReason::kHasBorderRadius);
+    mainThreadScrollingReasons |= MainThreadScrollingReason::kHasBorderRadius;
     needsCompositedScrolling = false;
   }
   if (layer->layoutObject()->hasClip() || layer->hasDescendantWithClipPath() ||
       layer->hasAncestorWithClipPath()) {
-    addStyleRelatedMainThreadScrollingReasons(
-        MainThreadScrollingReason::kHasClipRelatedProperty);
+    mainThreadScrollingReasons |=
+        MainThreadScrollingReason::kHasClipRelatedProperty;
     needsCompositedScrolling = false;
   }
+
+  if (mainThreadScrollingReasons) {
+    for (uint32_t reason = 1;

[bokan, 2017/01/23 19:55:51]

Just fold this loop into addStyleRelatedMainThreadScrollingReasons

[yigu, 2017/01/23 20:21:23]

Done.

+         reason <
+         1 << MainThreadScrollingReason::kMainThreadScrollingReasonCount;
+         reason <<= 1) {
+      if (mainThreadScrollingReasons & reason) {
+        addStyleRelatedMainThreadScrollingReasons(reason);
+      }
+    }
+  }
+
   return needsCompositedScrolling;
 }