Call scrollbarsChanged in PLSA::updateAfterStyleChange.

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 908 matching lines @@
   bool needsHorizontalScrollbar;
   bool needsVerticalScrollbar;
   // We add auto scrollbars only during layout to prevent spurious activations.
   computeScrollbarExistence(needsHorizontalScrollbar, needsVerticalScrollbar,
                             ForbidAddingAutoBars);
 
   // Avoid some unnecessary computation if there were and will be no scrollbars.
   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).
   if (hasHorizontalScrollbar() && oldStyle &&
       oldStyle->overflowX() == OverflowScroll &&
       box().style()->overflowX() != OverflowScroll) {
     horizontalScrollbar()->setEnabled(true);
   }
 
   if (hasVerticalScrollbar() && oldStyle &&
@@ skipping 226 matching lines @@
         frameView->calculateScrollbarModes(hMode, vMode);
         if (hMode == ScrollbarAlwaysOn)
           needsHorizontalScrollbar = true;
         if (vMode == ScrollbarAlwaysOn)
           needsVerticalScrollbar = true;
       }
     }
   }
 }
 
-void PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar) {
+bool PaintLayerScrollableArea::setHasHorizontalScrollbar(bool hasScrollbar) {
   if (FreezeScrollbarsScope::scrollbarsAreFrozen())
-    return;
+    return false;
 
   if (hasScrollbar == hasHorizontalScrollbar())
-    return;
+    return false;
 
   setScrollbarNeedsPaintInvalidation(HorizontalScrollbar);
 
   m_scrollbarManager.setHasHorizontalScrollbar(hasScrollbar);
 
   updateScrollOrigin();
 
   // Destroying or creating one bar can cause our scrollbar corner to come and
   // go. We need to update the opposite scrollbar's style.
   if (hasHorizontalScrollbar())
     horizontalScrollbar()->styleChanged();
   if (hasVerticalScrollbar())
     verticalScrollbar()->styleChanged();
 
   setScrollCornerNeedsPaintInvalidation();
 
   // 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);
 
   m_scrollbarManager.setHasVerticalScrollbar(hasScrollbar);
 
   updateScrollOrigin();
 
   // Destroying or creating one bar can cause our scrollbar corner to come and
   // go. We need to update the opposite scrollbar's style.
   if (hasHorizontalScrollbar())
     horizontalScrollbar()->styleChanged();
   if (hasVerticalScrollbar())
     verticalScrollbar()->styleChanged();
 
   setScrollCornerNeedsPaintInvalidation();
 
   // 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() &&
       (overlayScrollbarClipBehavior == IgnoreOverlayScrollbarSize ||
        !verticalScrollbar()->shouldParticipateInHitTesting()))
     return 0;
@@ skipping 763 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetsAfterLayout();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink