Don't update the logical width when a scrollbar gets added

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

 /*
  * 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, 2011 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. 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
@@ skipping 869 matching lines @@
         if (style()->isFlippedBlocksWritingMode()) {
             // FIXME: https://bugs.webkit.org/show_bug.cgi?id=97937
             // Workaround for now. We cannot delay the scroll info for overflow
             // for items with opposite writing directions, as the contents needs
             // to overflow in that direction
             layer()->scrollableArea()->updateAfterLayout();
             return;
         }
 
         if (gDelayUpdateScrollInfo) {
-            LayoutUnit logicalWidthExcludingScrollbar = logicalWidth() - scrollbarLogicalWidth();
             LayoutUnit logicalHeightExcludingScrollbar = logicalHeight() - scrollbarLogicalHeight();
             ScrollInfo scrollInfo;
             layer()->scrollableArea()->updateScrollDimensions(scrollInfo.scrollOffset, scrollInfo.autoHorizontalScrollBarChanged, scrollInfo.autoVerticalScrollBarChanged);
             DelayedUpdateScrollInfoMap::AddResult scrollInfoIterator = gDelayedUpdateScrollInfoMap->add(this, scrollInfo);
             if (!scrollInfoIterator.isNewEntry)
                 scrollInfoIterator.storedValue->value.merge(scrollInfo);
             if (scrollInfo.autoHorizontalScrollBarChanged)
                 setLogicalHeight(logicalHeightExcludingScrollbar + scrollbarLogicalHeight());

[skobes, 2015/11/18 22:45:24]

Is the setLogicalHeight call wrong for the same reason?

[cbiesinger, 2015/11/18 22:52:51]

So, that is a very good question and I was wondering about that. The short
answer is that I don't know. Since height is the *output* of layout it may be
OK.

No layout test fails if I do remove it...

So I think I'll remove it in a new patch (I want to land this one as soon as
possible due to the 4pm today deadline in bug 542197)

[skobes, 2015/11/18 23:01:55]

Width and height are both outputs of layout, and in general, neither should
change as scrollbars are added or removed.  Though I am not very familiar with
flexboxes and it's possible there are strange cases there.

-            if (scrollInfo.autoVerticalScrollBarChanged)
-                setLogicalWidth(logicalWidthExcludingScrollbar + scrollbarLogicalWidth());
         } else {
             layer()->scrollableArea()->updateAfterLayout();
         }
     }
 }
 
 void LayoutBlock::layout()
 {
     LayoutAnalyzer::Scope analyzer(*this);
 
@@ skipping 1986 matching lines @@
 void LayoutBlock::showLineTreeAndMark(const InlineBox* markedBox1, const char* markedLabel1, const InlineBox* markedBox2, const char* markedLabel2, const LayoutObject* obj) const
 {
     showLayoutObject();
     for (const RootInlineBox* root = firstRootBox(); root; root = root->nextRootBox())
         root->showLineTreeAndMark(markedBox1, markedLabel1, markedBox2, markedLabel2, obj, 1);
 }
 
 #endif
 
 } // namespace blink