Don't clamp scroll positions until the end of all layouts

third_party/WebKit/Source/core/layout/LayoutBlock.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 323 matching lines @@
     virtual bool shouldIgnoreOverflowPropertyForInlineBlockBaseline() const { return false; }
 
     void updateHitTestResult(HitTestResult&, const LayoutPoint&) override;
 
     // Delay update scrollbar until finishDelayUpdateScrollInfo() will be
     // called. This function is used when a flexbox is laying out its
     // descendant. If multiple calls are made to startDelayUpdateScrollInfo(),
     // finishDelayUpdateScrollInfo() will do nothing until finishDelayUpdateScrollInfo()
     // is called the same number of times.
     // finishDelayUpdateScrollInfo returns true when it marked something for layout.
+    // It will also return a map of saved scroll positions that the caller should restore
+    // on the given scrollable areas after performing the layout.
+    // This can be necessary because Flexbox's multi-pass layout can lose the scroll position.
+    typedef WTF::HashMap<PaintLayerScrollableArea*, DoublePoint> ScrollPositionMap;
     static void startDelayUpdateScrollInfo();
-    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*);
+    static bool finishDelayUpdateScrollInfo(SubtreeLayoutScope*, ScrollPositionMap*);
 
     void updateScrollInfoAfterLayout();
 
     void styleWillChange(StyleDifference, const ComputedStyle& newStyle) override;
     void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
     void updateFromStyle() override;
 
     // Returns true if non-visible overflow should be respected. Otherwise hasOverflowClip() will be
     // false and we won't create scrollable area for this object even if overflow is non-visible.
     virtual bool allowsOverflowClip() const;
@@ skipping 154 matching lines @@
     // FIXME: This is temporary as we move code that accesses block flow
     // member variables out of LayoutBlock and into LayoutBlockFlow.
     friend class LayoutBlockFlow;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBlock, isLayoutBlock());
 
 } // namespace blink
 
 #endif // LayoutBlock_h