Fix overflow:overlay scrollbar width for paint.

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 486 matching lines @@
 IntSize PaintLayerScrollableArea::minimumScrollOffsetInt() const {
   return toIntSize(-scrollOrigin());
 }
 
 IntSize PaintLayerScrollableArea::maximumScrollOffsetInt() const {
   if (!box().hasOverflowClip())
     return toIntSize(-scrollOrigin());
 
   IntSize contentSize = contentsSize();
   IntSize visibleSize =
-      pixelSnappedIntRect(box().overflowClipRect(box().location())).size();
+      pixelSnappedIntRect(
+          box().overflowClipRect(box().location(),
+                                 IgnorePlatformAndCSSOverlayScrollbarSize))

[pdr., 2017/03/08 20:33:20]

Is this the only callsite using IgnorePlatformAndCSSOverlayScrollbarSize? The
difference between IgnorePlatformAndCSSOverlayScrollbarSize and
IgnorePlatformOverlayScrollbarSize is subtle and it's not obvious which should
be used in which callsites. Is it possible to move the CSSOverlay code needed
for paint into a paint-specific function?

[szager1, 2017/03/08 20:43:09]

My initial patch did something like that, but skobes@ expressed a preference for
a new enum value.  So I will let the reviewers duke it out.

+          .size();
 
   FrameHost* host = layoutBox()->document().frameHost();
   DCHECK(host);
   TopDocumentRootScrollerController& controller =
       host->globalRootScrollerController();
 
   // The global root scroller should be clipped by the top FrameView rather
   // than it's overflow clipping box. This is to ensure that content exposed by
   // hiding the URL bar at the bottom of the screen is visible.
   if (this == controller.rootScrollerArea())
@@ skipping 771 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() ||
-       box().style()->overflowY() == EOverflow::kOverlay) &&
-      (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
-       !verticalScrollbar()->shouldParticipateInHitTesting())) {
+  if (overlayScrollbarClipBehavior ==
+          IgnorePlatformAndCSSOverlayScrollbarSize &&
+      box().style()->overflowY() == EOverflow::kOverlay) {
+    return 0;
+  }
+  if ((overlayScrollbarClipBehavior == IgnorePlatformOverlayScrollbarSize ||
+       overlayScrollbarClipBehavior ==
+           IgnorePlatformAndCSSOverlayScrollbarSize ||
+       !verticalScrollbar()->shouldParticipateInHitTesting()) &&
+      verticalScrollbar()->isOverlayScrollbar()) {
     return 0;
   }
   return verticalScrollbar()->scrollbarThickness();
 }
 
 int PaintLayerScrollableArea::horizontalScrollbarHeight(
     OverlayScrollbarClipBehavior overlayScrollbarClipBehavior) const {
   if (!hasHorizontalScrollbar())
     return 0;
-  if ((horizontalScrollbar()->isOverlayScrollbar() ||
-       box().style()->overflowX() == EOverflow::kOverlay) &&
-      (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
-       !horizontalScrollbar()->shouldParticipateInHitTesting())) {
+  if (overlayScrollbarClipBehavior ==
+          IgnorePlatformAndCSSOverlayScrollbarSize &&
+      box().style()->overflowX() == EOverflow::kOverlay) {
+    return 0;
+  }
+  if ((overlayScrollbarClipBehavior == IgnorePlatformOverlayScrollbarSize ||
+       overlayScrollbarClipBehavior ==
+           IgnorePlatformAndCSSOverlayScrollbarSize ||
+       !horizontalScrollbar()->shouldParticipateInHitTesting()) &&
+      horizontalScrollbar()->isOverlayScrollbar()) {
     return 0;
   }
   return horizontalScrollbar()->scrollbarThickness();
 }
 
 void PaintLayerScrollableArea::positionOverflowControls() {
   if (!hasScrollbar() && !box().canResize())
     return;
 
   const IntRect borderBox = box().pixelSnappedBorderBoxRect();
@@ skipping 804 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetAfterOverflowChange();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink