Make scroll anchoring adjustment when the bounds of a scroller is changed

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 734 matching lines @@
     // If a vertical scrollbar was removed, the min/max scroll positions may have changed,
     // so the scroll positions needs to be clamped.  If the scroll position did not change,
     // but the scroll origin *did* change, we still need to notify the scrollbars to
     // update their dimensions.
 
     if (DelayScrollPositionClampScope::clampingIsDelayed()) {
         DelayScrollPositionClampScope::setNeedsClamp(this);
         return;
     }
 
-    DoublePoint clampedScrollPosition = clampScrollPosition(scrollPositionDouble());
-    if (clampedScrollPosition != scrollPositionDouble())
-        ScrollableArea::setScrollPosition(clampedScrollPosition, ProgrammaticScroll);
-    else if (scrollOriginChanged())
-        scrollPositionChanged(clampedScrollPosition, ProgrammaticScroll);
+    DoublePoint clamped = clampScrollPosition(scrollPositionDouble());
+    if (clamped != scrollPositionDouble() || scrollOriginChanged()) {
+        // Since the scroll bounds have changed and the scroller is at a position
+        // that is outside its new bounds. Calling ScrollAnchor::restore
+        // ensures that no visible jump is seen when clamping.
+        if (shouldPerformScrollAnchoring() && m_scrollAnchor.hasScroller()) {
+            m_scrollAnchor.restore();
+            clamped = clampScrollPosition(scrollPositionDouble());
+        }
+        if (clamped != scrollPositionDouble())
+            ScrollableArea::setScrollPosition(clamped, ProgrammaticScroll);
+        resetScrollOriginChanged();
+    }
 
     setNeedsScrollPositionClamp(false);
-    resetScrollOriginChanged();
     m_scrollbarManager.destroyDetachedScrollbars();
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->getScrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
 {
@@ skipping 975 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink