Call scrollbarsChanged in PLSA::updateAfterStyleChange.

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 39afec388f4d7474c38c75129680c3a7ad0b2719..a2c0a5463f74cfc3c8c2cda56c0f111ab6041846 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -926,8 +926,15 @@ void PaintLayerScrollableArea::updateAfterStyleChange(
   if (!hasScrollbar() && !needsHorizontalScrollbar && !needsVerticalScrollbar)
     return;
 
-  setHasHorizontalScrollbar(needsHorizontalScrollbar);
-  setHasVerticalScrollbar(needsVerticalScrollbar);
+  bool horizontalScrollbarChanged =
+      setHasHorizontalScrollbar(needsHorizontalScrollbar);
+  bool verticalScrollbarChanged =
+      setHasVerticalScrollbar(needsVerticalScrollbar);
+
+  if (horizontalScrollbarChanged || verticalScrollbarChanged) {
+    toLayoutBlock(box()).scrollbarsChanged(horizontalScrollbarChanged,

[atotic, 2016/11/21 22:09:06]

box() can return null. Looking at existing code, it looks like everyone else
assumes it will never return null, so I guess it is ok.

box() is a LayoutBox. To typecast to LayoutBlock you need a guard:

   if (box().isLayoutBlock()) 

[skobes, 2016/11/21 22:18:51]

It returns a reference, so it cannot be null.
Done.

+                                           verticalScrollbarChanged);
+  }
 
   // With overflow: scroll, scrollbars are always visible but may be disabled.
   // When switching to another value, we need to re-enable them (see bug 11985).
@@ -1174,12 +1181,12 @@ void PaintLayerScrollableArea::computeScrollbarExistence(
   }
 }
 
-void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar) {
+bool PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar) {
   if (FreezeScrollbarsScope::scrollbarsAreFrozen())
-    return;
+    return false;
 
   if (hasScrollbar == hasHorizontalScrollbar())
-    return;
+    return false;
 
   setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
@@ -1199,14 +1206,15 @@ void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar) {
   // Force an update since we know the scrollbars have changed things.
   if (box().document().hasAnnotatedRegions())
     box().document().setAnnotatedRegionsDirty(true);
+  return true;
 }
 
-void PaintLayerScrollableArea::setHasVerticalScrollbar(bool hasScrollbar) {
+bool PaintLayerScrollableArea::setHasVerticalScrollbar(bool hasScrollbar) {
   if (FreezeScrollbarsScope::scrollbarsAreFrozen())
-    return;
+    return false;
 
   if (hasScrollbar == hasVerticalScrollbar())
-    return;
+    return false;
 
   setScrollbarNeedsPaintInvalidation(VerticalScrollbar);
 
@@ -1226,6 +1234,7 @@ void PaintLayerScrollableArea::setHasVerticalScrollbar(bool hasScrollbar) {
   // Force an update since we know the scrollbars have changed things.
   if (box().document().hasAnnotatedRegions())
     box().document().setAnnotatedRegionsDirty(true);
+  return true;
 }
 
 int PaintLayerScrollableArea::verticalScrollbarWidth(