Reland "Box shadow should be recorded as main thread scrolling reasons for non-root layers."

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 d025478fc3d914414951d35431bf6274eaa682cf..8eef9f10843509642a26e3d269a9b514b57ba9c3 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -1750,13 +1750,15 @@
     return false;
 
   bool needsCompositedScrolling = true;
+  uint32_t mainThreadScrollingReasons = 0;
 
   // 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->backgroundPaintLocation(&mainThreadScrollingReasons) &
+          BackgroundPaintInScrollingContents &&
       layer->backgroundIsKnownToBeOpaqueInRect(
           toLayoutBox(layer->layoutObject())->paddingBoxRect()) &&
       !layer->compositesWithTransform() && !layer->compositesWithOpacity();
@@ -1765,18 +1767,19 @@
       !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;
+    }
+
     needsCompositedScrolling = false;
   }
 
@@ -1785,21 +1788,25 @@
   // 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) {
+    addStyleRelatedMainThreadScrollingReasons(mainThreadScrollingReasons);
+  }
+
   return needsCompositedScrolling;
 }
 
 void PaintLayerScrollableArea::addStyleRelatedMainThreadScrollingReasons(
-    const uint32_t reason) {
+    const uint32_t reasons) {
   LocalFrame* frame = box().frame();
   if (!frame)
     return;
@@ -1807,8 +1814,14 @@
   if (!frameView)
     return;
 
-  frameView->adjustStyleRelatedMainThreadScrollingReasons(reason, true);
-  m_reasons |= reason;
+  for (uint32_t reason = 1;
+       reason < 1 << MainThreadScrollingReason::kMainThreadScrollingReasonCount;
+       reason <<= 1) {
+    if (reasons & reason) {
+      frameView->adjustStyleRelatedMainThreadScrollingReasons(reason, true);
+      m_reasons |= reason;
+    }
+  }
 }
 
 void PaintLayerScrollableArea::removeStyleRelatedMainThreadScrollingReasons() {