Fix overflow:overlay scrollbar width for paint.

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 8a5fd7f3bc135dc51d8b1fe6a2e3615ff0f209be..8c71e109d785caac9c34f54e2adf271af7ea3cbe 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -506,6 +506,15 @@ IntSize PaintLayerScrollableArea::maximumScrollOffsetInt() const {
   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 =
@@ -1296,12 +1305,10 @@ 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();
 }
 
@@ -1309,12 +1316,10 @@ 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();
 }