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();

[skobes, 2017/03/07 16:42:47]

Could we add a value to the OverlayScrollbarClipBehavior enum indicating that
both regular-overlay and overflow:overlay scrollbars should be ignored, and pass
it here?

[szager1, 2017/03/07 19:01:31]

Yeah, that was my first attempt.  It works, but it's a more invasive change.  If
you prefer that, I'll upload that version.

[skobes, 2017/03/07 19:09:12]

I probably do, though I'll have to see the patch. :)

If we have existing plumbing to conditionally subtract scrollbar space, it's
cleaner to refine the conditions than to "undo" the subtraction after the fact.

 
+  // When computing scroll limits, overflow:overlay scrollbars should not be
+  // subtracted from the visible content size.
+  if (hasVerticalScrollbar() &&
+      box().style()->overflowY() == EOverflow::kOverlay)
+    visibleSize.expand(verticalScrollbar()->scrollbarThickness(), 0);
+  if (hasHorizontalScrollbar() &&
+      box().style()->overflowX() == EOverflow::kOverlay)
+    visibleSize.expand(0, horizontalScrollbar()->scrollbarThickness());
+
   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())
     visibleSize = controller.rootScrollerVisibleArea();
@@ skipping 770 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) &&
+  if (verticalScrollbar()->isOverlayScrollbar() &&
       (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
-       !verticalScrollbar()->shouldParticipateInHitTesting())) {
+       !verticalScrollbar()->shouldParticipateInHitTesting()))
     return 0;
-  }
   return verticalScrollbar()->scrollbarThickness();
 }
 
 int PaintLayerScrollableArea::horizontalScrollbarHeight(
     OverlayScrollbarClipBehavior overlayScrollbarClipBehavior) const {
   if (!hasHorizontalScrollbar())
     return 0;
-  if ((horizontalScrollbar()->isOverlayScrollbar() ||
-       box().style()->overflowX() == EOverflow::kOverlay) &&
+  if (horizontalScrollbar()->isOverlayScrollbar() &&
       (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