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

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

Index: third_party/WebKit/Source/core/layout/LayoutBlock.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutBlock.cpp b/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
index ba3abb1d3659862d6093f1c0ed478e8b6b7de214..7c513b92a0dae30daa3008661ca25c9351bc4f7a 100644
--- a/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutBlock.cpp
@@ -887,7 +887,6 @@ void LayoutBlock::updateScrollInfoAfterLayout()
         }
 
         if (gDelayUpdateScrollInfo) {
-            LayoutUnit logicalWidthExcludingScrollbar = logicalWidth() - scrollbarLogicalWidth();
             LayoutUnit logicalHeightExcludingScrollbar = logicalHeight() - scrollbarLogicalHeight();
             ScrollInfo scrollInfo;
             layer()->scrollableArea()->updateScrollDimensions(scrollInfo.scrollOffset, scrollInfo.autoHorizontalScrollBarChanged, scrollInfo.autoVerticalScrollBarChanged);
@@ -896,8 +895,6 @@ void LayoutBlock::updateScrollInfoAfterLayout()
                 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();
         }