Account for overlay scrollbars when computing scrollbar width/height.

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 1266 matching lines @@
   // Force an update since we know the scrollbars have changed things.
   if (box().document().hasAnnotatedRegions())
     box().document().setAnnotatedRegionsDirty(true);
   return true;
 }
 
 int PaintLayerScrollableArea::verticalScrollbarWidth(
     OverlayScrollbarClipBehavior overlayScrollbarClipBehavior) const {
   if (!hasVerticalScrollbar())
     return 0;
-  if (verticalScrollbar()->isOverlayScrollbar() &&
+  if ((verticalScrollbar()->isOverlayScrollbar() ||
+       box().style()->overflowY() == EOverflow::kOverlay) &&
       (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
-       !verticalScrollbar()->shouldParticipateInHitTesting()))
+       !verticalScrollbar()->shouldParticipateInHitTesting())) {
     return 0;
+  }
   return verticalScrollbar()->scrollbarThickness();
 }
 
 int PaintLayerScrollableArea::horizontalScrollbarHeight(
     OverlayScrollbarClipBehavior overlayScrollbarClipBehavior) const {
   if (!hasHorizontalScrollbar())
     return 0;
-  if (horizontalScrollbar()->isOverlayScrollbar() &&
+  if ((horizontalScrollbar()->isOverlayScrollbar() ||
+       box().style()->overflowX() == EOverflow::kOverlay) &&
       (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
-       !horizontalScrollbar()->shouldParticipateInHitTesting()))
+       !horizontalScrollbar()->shouldParticipateInHitTesting())) {
     return 0;
+  }
   return horizontalScrollbar()->scrollbarThickness();
 }
 
 void PaintLayerScrollableArea::positionOverflowControls() {
   if (!hasScrollbar() && !box().canResize())
     return;
 
   const IntRect borderBox = box().pixelSnappedBorderBoxRect();
   if (Scrollbar* verticalScrollbar = this->verticalScrollbar())
     verticalScrollbar->setFrameRect(rectForVerticalScrollbar(borderBox));
@@ skipping 802 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink