Restore PaintLayerScrollableArea::ScrollbarManager::canDetach behavior.

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>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 743 matching lines @@
     }
 
     DoublePoint clampedScrollPosition = clampScrollPosition(scrollPositionDouble());
     if (clampedScrollPosition != scrollPositionDouble())
         ScrollableArea::setScrollPosition(clampedScrollPosition, ProgrammaticScroll);
     else if (scrollOriginChanged())
         scrollPositionChanged(clampedScrollPosition, ProgrammaticScroll);
 
     setNeedsScrollPositionClamp(false);
     resetScrollOriginChanged();
+    m_scrollbarManager.destroyDetachedScrollbars();
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->getScrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
 {
     return pixelSnappedScrollWidth() > box().pixelSnappedClientWidth();
@@ skipping 796 matching lines @@
         if (!m_hBar) {
             m_hBar = createScrollbar(HorizontalScrollbar);
             m_hBarIsAttached = 1;
             if (!m_hBar->isCustomScrollbar())
                 m_scrollableArea->didAddScrollbar(*m_hBar, HorizontalScrollbar);
         } else {
             m_hBarIsAttached = 1;
         }
     } else {
         m_hBarIsAttached = 0;
-        destroyScrollbar(HorizontalScrollbar);
+        if (!DelayScrollPositionClampScope::clampingIsDelayed())
+            destroyScrollbar(HorizontalScrollbar);
     }
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::setHasVerticalScrollbar(bool hasScrollbar)
 {
     if (hasScrollbar) {
         DisableCompositingQueryAsserts disabler;
         if (!m_vBar) {
             m_vBar = createScrollbar(VerticalScrollbar);
             m_vBarIsAttached = 1;
             if (!m_vBar->isCustomScrollbar())
                 m_scrollableArea->didAddScrollbar(*m_vBar, VerticalScrollbar);
         } else {
             m_vBarIsAttached = 1;
         }
     } else {
         m_vBarIsAttached = 0;
-        destroyScrollbar(VerticalScrollbar);
+        if (!DelayScrollPositionClampScope::clampingIsDelayed())
+            destroyScrollbar(VerticalScrollbar);
     }
 }
 
 Scrollbar* PaintLayerScrollableArea::ScrollbarManager::createScrollbar(ScrollbarOrientation orientation)
 {
     ASSERT(orientation == HorizontalScrollbar ? !m_hBarIsAttached : !m_vBarIsAttached);
     Scrollbar* scrollbar = nullptr;
     const LayoutObject& actualLayoutObject = layoutObjectForScrollbar(m_scrollableArea->box());
     bool hasCustomScrollbarStyle = actualLayoutObject.isBox() && actualLayoutObject.styleRef().hasPseudoStyle(PseudoIdScrollbar);
     if (hasCustomScrollbarStyle) {
@@ skipping 109 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink