Don't adjust for scrolling of the ancestor, rather than un-adjusting for it later.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 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 32 matching lines @@
 enum AvailableLogicalHeightType { ExcludeMarginBorderPadding, IncludeMarginBorderPadding };
 // When painting, overlay scrollbars do not take up space and should not affect
 // clipping behavior. During hit testing, overlay scrollbars behave like regular
 // scrollbars and should change how hit testing is clipped.
 enum MarginDirection { BlockDirection, InlineDirection };
 
 enum ShouldComputePreferred { ComputeActual, ComputePreferred };
 
 enum ApplyOverflowClipFlag {
     ApplyOverflowClip,
+    // Don't apply overflow clipping or scrolling.
     ApplyNonScrollOverflowClip
 };
 
 using SnapAreaSet = HashSet<const LayoutBox*>;
 
 struct LayoutBoxRareData {
     WTF_MAKE_NONCOPYABLE(LayoutBoxRareData); USING_FAST_MALLOC(LayoutBoxRareData);
 public:
     LayoutBoxRareData()
         : m_spannerPlaceholder(nullptr)
@@ skipping 1171 matching lines @@
         || breakValue == BreakLeft
         || breakValue == BreakPage
         || breakValue == BreakRecto
         || breakValue == BreakRight
         || breakValue == BreakVerso;
 }
 
 } // namespace blink
 
 #endif // LayoutBox_h