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..1ed376f5621eba05f543b35248939684a3c76b40 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -504,7 +504,10 @@ IntSize PaintLayerScrollableArea::maximumScrollOffsetInt() const {
 
   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);
@@ -1296,10 +1299,16 @@ 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();
@@ -1309,10 +1318,16 @@ 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();