If the content area is given an overflow a { width: 100%; border: 1px solid black; } has the effect of creating a block which is always 2px wider than the space that the scrolling area can contain and hence creates a horizontal scrollbar that never disappears.
In various places we use width: 100%; and give something a border. We should fix these places by either eliminating the width: 100%; (if it's unnecessary for that area to have it), or by specifying a box-sizing: border-box; on that element (with all the fallbacks) so that the width is calculated correctly in supporting browsers.
Note that this issue does not happen with tables, so we don't need to deal with all those people put inside of WikiText. Just ui css we add.
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