Fix overflow:overlay scrollbar width for paint.

Re-landing this change:

https://codereview.chromium.org/2730343003/

... along with the missing test expectations.

TBR=tzik@chromium.org
BUG=697751
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2757533002
Cr-Commit-Position: refs/heads/master@{#457353}

third_party/WebKit/LayoutTests/platform/linux/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 284.00 scrollHeight 842
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 284.00 scrollHeight 842
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-242) size 559x842 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-242) size 559x842 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 558.97x842
     LayoutBlockFlow (anonymous) at (0,0) size 558.97x20
       LayoutText {#text} at (0,0) size 559x19

third_party/WebKit/LayoutTests/platform/mac-mac10.10/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 278.00 scrollHeight 836
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 278.00 scrollHeight 836
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-236) size 570x836 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-236) size 570x836 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 570.41x836
     LayoutBlockFlow (anonymous) at (0,0) size 570.41x18
       LayoutText {#text} at (0,0) size 571x18

third_party/WebKit/LayoutTests/platform/mac-mac10.11/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 278.00 scrollHeight 836
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 278.00 scrollHeight 836
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-236) size 570x836 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-236) size 570x836 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 570.41x836
     LayoutBlockFlow (anonymous) at (0,0) size 570.41x18
       LayoutText {#text} at (0,0) size 571x18

third_party/WebKit/LayoutTests/platform/mac-mac10.9/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 278.00 scrollHeight 836
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 278.00 scrollHeight 836
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-236) size 570x836 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-236) size 570x836 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 570.41x836
     LayoutBlockFlow (anonymous) at (0,0) size 570.41x18
       LayoutText {#text} at (0,0) size 571x18

third_party/WebKit/LayoutTests/platform/mac-retina/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 278.00 scrollHeight 836
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 278.00 scrollHeight 836
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-236) size 570x836 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-236) size 570x836 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 570.41x836
     LayoutBlockFlow (anonymous) at (0,0) size 570.41x18
       LayoutText {#text} at (0,0) size 571x18

third_party/WebKit/LayoutTests/platform/mac/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 278.00 scrollHeight 836
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 278.00 scrollHeight 836
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-236) size 570x836 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-236) size 570x836 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 570.41x836
     LayoutBlockFlow (anonymous) at (0,0) size 570.41x18
       LayoutText {#text} at (0,0) size 571x18

third_party/WebKit/LayoutTests/platform/win/fast/overflow/scroll-nested-positioned-layer-in-overflow-expected.txt

@@ -3,9 +3,9 @@ layer at (0,0) size 800x600
 layer at (0,0) size 800x600
   LayoutBlockFlow {HTML} at (0,0) size 800x600
     LayoutBlockFlow {BODY} at (8,8) size 784x584
-layer at (0,42) size 800x558 scrollY 282.00 scrollHeight 840
+layer at (0,42) size 800x558 clip at (0,42) size 785x558 scrollY 282.00 scrollHeight 840
   LayoutBlockFlow (positioned) {DIV} at (0,42) size 800x558
-layer at (0,-240) size 571x840 backgroundClip at (0,42) size 800x558 clip at (0,42) size 800x558
+layer at (0,-240) size 571x840 backgroundClip at (0,42) size 785x558 clip at (0,42) size 785x558
   LayoutBlockFlow (positioned) {DIV} at (0,0) size 571.28x840
     LayoutBlockFlow (anonymous) at (0,0) size 571.28x18
       LayoutText {#text} at (0,0) size 572x17

third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/overflow-overlay-with-touch-expected.txt

@@ -15,7 +15,7 @@
     },
     {
       "name": "Scrolling Layer",
-      "bounds": [300, 300],
+      "bounds": [285, 285],
       "shouldFlattenTransform": false
     },
     {

third_party/WebKit/Source/core/frame/RootFrameViewport.h

@@ -86,9 +86,11 @@ class CORE_EXPORT RootFrameViewport final
   GraphicsLayer* layerForVerticalScrollbar() const override;
   GraphicsLayer* layerForScrollCorner() const override;
   int horizontalScrollbarHeight(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
   int verticalScrollbarWidth(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
   ScrollResult userScroll(ScrollGranularity, const FloatSize&) override;
   bool scrollAnimatorEnabled() const override;
   HostWindow* getHostWindow() const override;

third_party/WebKit/Source/core/layout/LayoutBox.h

@@ -1088,7 +1088,7 @@ class CORE_EXPORT LayoutBox : public LayoutBoxModelObject {
   // Returns the intersection of all overflow clips which apply.
   virtual LayoutRect overflowClipRect(
       const LayoutPoint& location,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const;
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize) const;
   LayoutRect clipRect(const LayoutPoint& location) const;
 
   // Returns the combination of overflow clip, contain: paint clip and CSS clip
@@ -1445,7 +1445,7 @@ class CORE_EXPORT LayoutBox : public LayoutBoxModelObject {
 
   void excludeScrollbars(
       LayoutRect&,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const;
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize) const;
 
   LayoutUnit containingBlockLogicalWidthForPositioned(
       const LayoutBoxModelObject* containingBlock,

third_party/WebKit/Source/core/layout/LayoutTable.h

@@ -508,7 +508,8 @@ class CORE_EXPORT LayoutTable final : public LayoutBlock {
 
   LayoutRect overflowClipRect(
       const LayoutPoint& location,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
 
   void addOverflowFromChildren() override;
 

third_party/WebKit/Source/core/layout/LayoutView.h

@@ -173,7 +173,8 @@ class CORE_EXPORT LayoutView final : public LayoutBlockFlow {
   LayoutRect viewRect() const override;
   LayoutRect overflowClipRect(
       const LayoutPoint& location,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
 
   LayoutState* layoutState() const { return m_layoutState; }
 

third_party/WebKit/Source/core/layout/api/LayoutBoxItem.h

@@ -49,7 +49,7 @@ class LayoutBoxItem : public LayoutBoxModel {
 
   LayoutRect overflowClipRect(const LayoutPoint& location,
                               OverlayScrollbarClipBehavior behavior =
-                                  IgnoreOverlayScrollbarSize) const {
+                                  IgnorePlatformOverlayScrollbarSize) const {
     return toBox()->overflowClipRect(location, behavior);
   }
 

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

@@ -550,7 +550,7 @@ void CompositedLayerMapping::
   // FIXME: this should use cached clip rects, but this sometimes give
   // inaccurate results (and trips the ASSERTS in PaintLayerClipper).
   ClipRectsContext clipRectsContext(compositingAncestor, UncachedClipRects,
-                                    IgnoreOverlayScrollbarSize);
+                                    IgnorePlatformOverlayScrollbarSize);
   clipRectsContext.setIgnoreOverflowClip();
 
   ClipRect clipRect;
@@ -1162,7 +1162,7 @@ void CompositedLayerMapping::updateAncestorClippingLayerGeometry(
 
   ClipRectsContext clipRectsContext(compositingContainer,
                                     PaintingClipRectsIgnoringOverflowClip,
-                                    IgnoreOverlayScrollbarSize);
+                                    IgnorePlatformOverlayScrollbarSize);
 
   ClipRect parentClipRect;
   m_owningLayer.clipper(PaintLayer::DoNotUseGeometryMapper)

third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.cpp

@@ -230,8 +230,8 @@ LayoutRect LayoutSVGRoot::visualOverflowRect() const {
 
 LayoutRect LayoutSVGRoot::overflowClipRect(const LayoutPoint& location,
                                            OverlayScrollbarClipBehavior) const {
-  return LayoutRect(pixelSnappedIntRect(
-      LayoutReplaced::overflowClipRect(location, IgnoreOverlayScrollbarSize)));
+  return LayoutRect(pixelSnappedIntRect(LayoutReplaced::overflowClipRect(
+      location, IgnorePlatformOverlayScrollbarSize)));
 }
 
 void LayoutSVGRoot::paintReplaced(const PaintInfo& paintInfo,

third_party/WebKit/Source/core/layout/svg/LayoutSVGRoot.h

@@ -90,7 +90,8 @@ class CORE_EXPORT LayoutSVGRoot final : public LayoutReplaced {
   LayoutRect visualOverflowRect() const override;
   LayoutRect overflowClipRect(
       const LayoutPoint& location,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
 
   bool hasNonIsolatedBlendingDescendants() const final;
 

third_party/WebKit/Source/core/paint/ClipRectsCache.h

@@ -41,7 +41,7 @@ class ClipRectsCache {
         : root(nullptr)
 #if DCHECK_IS_ON()
           ,
-          overlayScrollbarClipBehavior(IgnoreOverlayScrollbarSize)
+          overlayScrollbarClipBehavior(IgnorePlatformOverlayScrollbarSize)
 #endif
     {
     }

third_party/WebKit/Source/core/paint/PaintLayer.h

@@ -845,7 +845,7 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
       const LayoutRect& dirtyRect,
       ClipRectsCacheSlot,
       GeometryMapperOption,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize,
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize,
       ShouldRespectOverflowClipType = RespectOverflowClip,
       const LayoutPoint* offsetFromRoot = 0,
       const LayoutSize& subPixelAccumulation = LayoutSize());
@@ -855,7 +855,7 @@ class CORE_EXPORT PaintLayer : public DisplayItemClient {
       const LayoutRect& dirtyRect,
       ClipRectsCacheSlot,
       GeometryMapperOption,
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize,
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize,
       ShouldRespectOverflowClipType = RespectOverflowClip,
       const LayoutPoint* offsetFromRoot = 0,
       const LayoutSize& subPixelAccumulation = LayoutSize(),

third_party/WebKit/Source/core/paint/PaintLayerClipper.cpp

@@ -588,7 +588,8 @@ ClipRects& PaintLayerClipper::paintingClipRects(
     const LayoutSize& subpixelAccumulation) const {
   DCHECK(!m_geometryMapper);
   ClipRectsContext context(rootLayer, PaintingClipRects,
-                           IgnoreOverlayScrollbarSize, subpixelAccumulation);
+                           IgnorePlatformOverlayScrollbarSize,
+                           subpixelAccumulation);
   if (respectOverflowClip == IgnoreOverflowClip)
     context.setIgnoreOverflowClip();
   return getClipRects(context);

third_party/WebKit/Source/core/paint/PaintLayerClipper.h

@@ -66,7 +66,7 @@ class ClipRectsContext {
   ClipRectsContext(const PaintLayer* root,
                    ClipRectsCacheSlot slot,
                    OverlayScrollbarClipBehavior overlayScrollbarClipBehavior =
-                       IgnoreOverlayScrollbarSize,
+                       IgnorePlatformOverlayScrollbarSize,
                    const LayoutSize& accumulation = LayoutSize())
       : rootLayer(root),
         overlayScrollbarClipBehavior(overlayScrollbarClipBehavior),

third_party/WebKit/Source/core/paint/PaintLayerClipperTest.cpp

@@ -48,7 +48,7 @@ TEST_P(PaintLayerClipperTest, LayoutSVGRoot) {
   PaintLayer* targetPaintLayer =
       toLayoutBoxModelObject(target->layoutObject())->layer();
   ClipRectsContext context(document().layoutView()->layer(), UncachedClipRects,
-                           IgnoreOverlayScrollbarSize,
+                           IgnorePlatformOverlayScrollbarSize,
                            LayoutSize(FloatSize(0.25, 0.35)));
   // When RLS is enabled, the LayoutView will have a composited scrolling layer,
   // so don't apply an overflow clip.

third_party/WebKit/Source/core/paint/PaintLayerPainter.cpp

@@ -450,23 +450,23 @@ PaintResult PaintLayerPainter::paintLayerContents(
       paintLayerForFragments->appendSingleFragmentIgnoringPagination(
           layerFragments, localPaintingInfo.rootLayer,
           localPaintingInfo.paintDirtyRect, cacheSlot, geometryMapperOption,
-          IgnoreOverlayScrollbarSize, respectOverflowClip, &offsetFromRoot,
-          localPaintingInfo.subPixelAccumulation);
+          IgnorePlatformOverlayScrollbarSize, respectOverflowClip,
+          &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
     } else if (isFixedPositionObjectInPagedMedia()) {
       PaintLayerFragments singleFragment;
       paintLayerForFragments->appendSingleFragmentIgnoringPagination(
           singleFragment, localPaintingInfo.rootLayer,
           localPaintingInfo.paintDirtyRect, cacheSlot, geometryMapperOption,
-          IgnoreOverlayScrollbarSize, respectOverflowClip, &offsetFromRoot,
-          localPaintingInfo.subPixelAccumulation);
+          IgnorePlatformOverlayScrollbarSize, respectOverflowClip,
+          &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
       repeatFixedPositionObjectInPages(singleFragment[0], paintingInfo,
                                        layerFragments);
     } else {
       paintLayerForFragments->collectFragments(
           layerFragments, localPaintingInfo.rootLayer,
           localPaintingInfo.paintDirtyRect, cacheSlot, geometryMapperOption,
-          IgnoreOverlayScrollbarSize, respectOverflowClip, &offsetFromRoot,
-          localPaintingInfo.subPixelAccumulation);
+          IgnorePlatformOverlayScrollbarSize, respectOverflowClip,
+          &offsetFromRoot, localPaintingInfo.subPixelAccumulation);
     }
 
     if (paintFlags & PaintLayerPaintingAncestorClippingMaskPhase) {
@@ -729,7 +729,7 @@ PaintResult PaintLayerPainter::paintLayerWithTransform(
     // here.
     paginationLayer->collectFragments(
         layerFragments, paintingInfo.rootLayer, paintingInfo.paintDirtyRect,
-        cacheSlot, geometryMapperOption, IgnoreOverlayScrollbarSize,
+        cacheSlot, geometryMapperOption, IgnorePlatformOverlayScrollbarSize,
         respectOverflowClip, nullptr, paintingInfo.subPixelAccumulation,
         &transformedExtent);
   }
@@ -746,7 +746,7 @@ PaintResult PaintLayerPainter::paintLayerWithTransform(
           paintingInfo.rootLayer,
           (paintFlags & PaintLayerUncachedClipRects) ? UncachedClipRects
                                                      : PaintingClipRects,
-          IgnoreOverlayScrollbarSize);
+          IgnorePlatformOverlayScrollbarSize);
       if (shouldRespectOverflowClip(paintFlags, m_paintLayer.layoutObject()) ==
           IgnoreOverflowClip)
         clipRectsContext.setIgnoreOverflowClip();

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -506,7 +506,10 @@ IntSize PaintLayerScrollableArea::maximumScrollOffsetInt() const {
 
   IntSize contentSize = contentsSize();
   IntSize visibleSize =
-      pixelSnappedIntRect(box().overflowClipRect(box().location())).size();
+      pixelSnappedIntRect(
+          box().overflowClipRect(box().location(),
+                                 IgnorePlatformAndCSSOverlayScrollbarSize))
+          .size();
 
   Page* page = layoutBox()->document().page();
   DCHECK(page);
@@ -1300,10 +1303,16 @@ 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();
@@ -1313,10 +1322,16 @@ 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();

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h

@@ -356,9 +356,11 @@ class CORE_EXPORT PaintLayerScrollableArea final
   int pixelSnappedScrollHeight() const;
 
   int verticalScrollbarWidth(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
   int horizontalScrollbarHeight(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const override;
+      OverlayScrollbarClipBehavior =
+          IgnorePlatformOverlayScrollbarSize) const override;
 
   DoubleSize adjustedScrollOffset() const {
     return toDoubleSize(DoublePoint(scrollOrigin()) + m_scrollOffset);

third_party/WebKit/Source/platform/scroll/ScrollTypes.h

@@ -43,8 +43,12 @@ inline ScrollOffset toScrollOffset(const FloatPoint& p) {
   return ScrollOffset(p.x(), p.y());
 }
 
+// Platform overlay scrollbars are controlled and painted by the operating
+// system (e.g., OSX and Android).  CSS overlay scrollbars are created by
+// setting overflow:overlay, and they are painted by chromium.
 enum OverlayScrollbarClipBehavior {
-  IgnoreOverlayScrollbarSize,
+  IgnorePlatformOverlayScrollbarSize,
+  IgnorePlatformAndCSSOverlayScrollbarSize,
   ExcludeOverlayScrollbarSizeForHitTesting
 };
 

third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp

@@ -635,7 +635,7 @@ float ScrollableArea::pixelStep(ScrollbarOrientation) const {
 
 int ScrollableArea::verticalScrollbarWidth(
     OverlayScrollbarClipBehavior behavior) const {
-  DCHECK_EQ(behavior, IgnoreOverlayScrollbarSize);
+  DCHECK_EQ(behavior, IgnorePlatformOverlayScrollbarSize);
   if (Scrollbar* verticalBar = verticalScrollbar())
     return !verticalBar->isOverlayScrollbar() ? verticalBar->width() : 0;
   return 0;
@@ -643,7 +643,7 @@ int ScrollableArea::verticalScrollbarWidth(
 
 int ScrollableArea::horizontalScrollbarHeight(
     OverlayScrollbarClipBehavior behavior) const {
-  DCHECK_EQ(behavior, IgnoreOverlayScrollbarSize);
+  DCHECK_EQ(behavior, IgnorePlatformOverlayScrollbarSize);
   if (Scrollbar* horizontalBar = horizontalScrollbar())
     return !horizontalBar->isOverlayScrollbar() ? horizontalBar->height() : 0;
   return 0;

third_party/WebKit/Source/platform/scroll/ScrollableArea.h

@@ -335,9 +335,9 @@ class PLATFORM_EXPORT ScrollableArea : public GarbageCollectedMixin,
   IntSize excludeScrollbars(const IntSize&) const;
 
   virtual int verticalScrollbarWidth(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const;
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize) const;
   virtual int horizontalScrollbarHeight(
-      OverlayScrollbarClipBehavior = IgnoreOverlayScrollbarSize) const;
+      OverlayScrollbarClipBehavior = IgnorePlatformOverlayScrollbarSize) const;
 
   // Returns the widget associated with this ScrollableArea.
   virtual FrameViewBase* getFrameViewBase() { return nullptr; }