Record non-stacking-context 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 1834 matching lines @@
     }
     if (layer->CompositesWithTransform()) {
       non_composited_main_thread_scrolling_reasons_ |=
           MainThreadScrollingReason::kHasTransformAndLCDText;
     }
     if (!layer->BackgroundIsKnownToBeOpaqueInRect(
             ToLayoutBox(layer->GetLayoutObject()).PaddingBoxRect())) {
       non_composited_main_thread_scrolling_reasons_ |=
           MainThreadScrollingReason::kBackgroundNotOpaqueInRectAndLCDText;
     }
+    if (!layer->GetLayoutObject().Style()->IsStackingContext()) {
+      non_composited_main_thread_scrolling_reasons_ |=
+          MainThreadScrollingReason::kIsNotStackingContextAndLCDText;
+    }
 
     needs_composited_scrolling = 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->GetLayoutObject().Style()->HasBorderRadius()) {
     non_composited_main_thread_scrolling_reasons_ |=
@@ skipping 284 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     ClampScrollableAreas() {
   for (auto& scrollable_area : *needs_clamp_)
     scrollable_area->ClampScrollOffsetAfterOverflowChange();
   delete needs_clamp_;
   needs_clamp_ = nullptr;
 }
 
 }  // namespace blink