Disable overlay scrollbars in Blink when hidden by the compositor.

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 634 matching lines @@
   updateScrollOrigin();
 }
 
 void PaintLayerScrollableArea::setScrollOffsetUnconditionally(
     const ScrollOffset& offset,
     ScrollType scrollType) {
   cancelScrollAnimation();
   scrollOffsetChanged(offset, scrollType);
 }
 
+void PaintLayerScrollableArea::didChangeScrollbarsHidden() {
+  updateScrollbarsEnabledState();
+}
+
+void PaintLayerScrollableArea::updateScrollbarsEnabledState() {
+  // overflow:scroll should just enable/disable.
+  if (box().style()->overflowX() == OverflowScroll && horizontalScrollbar()) {
+    horizontalScrollbar()->setEnabled(hasHorizontalOverflow() &&
+                                      !scrollbarsHidden());
+  }
+  if (box().style()->overflowY() == OverflowScroll && verticalScrollbar()) {
+    verticalScrollbar()->setEnabled(hasVerticalOverflow() &&
+                                    !scrollbarsHidden());
+  }
+}
+
 void PaintLayerScrollableArea::updateAfterLayout() {
   ASSERT(box().hasOverflowClip());
 
   bool relayoutIsPrevented = PreventRelayoutScope::relayoutIsPrevented();
   bool scrollbarsAreFrozen =
       m_inOverflowRelayout || FreezeScrollbarsScope::scrollbarsAreFrozen();
 
   if (needsScrollbarReconstruction()) {
     setHasHorizontalScrollbar(false);
     setHasVerticalScrollbar(false);
@@ skipping 66 matching lines @@
       if (parent && parent->isFlexibleBox())
         toLayoutFlexibleBox(parent)->clearCachedMainSizeForChild(box());
     }
   }
 
   {
     // Hits in
     // compositing/overflow/automatically-opt-into-composited-scrolling-after-style-change.html.
     DisableCompositingQueryAsserts disabler;
 
-    // overflow:scroll should just enable/disable.
-    if (box().style()->overflowX() == OverflowScroll && horizontalScrollbar())
-      horizontalScrollbar()->setEnabled(hasHorizontalOverflow());
-    if (box().style()->overflowY() == OverflowScroll && verticalScrollbar())
-      verticalScrollbar()->setEnabled(hasVerticalOverflow());
+    updateScrollbarsEnabledState();
 
     // Set up the range (and page step/line step).
     if (Scrollbar* horizontalScrollbar = this->horizontalScrollbar()) {
       int clientWidth = box().pixelSnappedClientWidth();
       horizontalScrollbar->setProportion(clientWidth,
                                          overflowRect().width().toInt());
     }
     if (Scrollbar* verticalScrollbar = this->verticalScrollbar()) {
       int clientHeight = box().pixelSnappedClientHeight();
       verticalScrollbar->setProportion(clientHeight,
@@ skipping 1193 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetsAfterLayout();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink