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 1738 matching lines @@
   if (node && node->isElementNode() &&
       (toElement(node)->compositorMutableProperties() &
        (CompositorMutableProperty::kScrollTop |
         CompositorMutableProperty::kScrollLeft)))
     return true;
 
   if (layer->size().isEmpty())
     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.
   bool backgroundSupportsLCDText =
       RuntimeEnabledFeatures::compositeOpaqueScrollersEnabled() &&
       layer->layoutObject()->style()->isStackingContext() &&
       layer->backgroundPaintLocation() & BackgroundPaintInScrollingContents &&
       layer->backgroundIsKnownToBeOpaqueInRect(
           toLayoutBox(layer->layoutObject())->paddingBoxRect()) &&
       !layer->compositesWithTransform() && !layer->compositesWithOpacity();
 
   if (mode == PaintLayerScrollableArea::ConsiderLCDText &&
       !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;
   }
 
   // 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);
+    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;
 }
 
 void PaintLayerScrollableArea::addStyleRelatedMainThreadScrollingReasons(
     const uint32_t reason) {
   LocalFrame* frame = box().frame();
   if (!frame)
     return;
   FrameView* frameView = frame->view();
   if (!frameView)
@@ skipping 290 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink