Horizontal / vertical scrollbars render a 'corner' on the lower right
when they would otherwise overlap. I previously relied on drawing both
dummy scrollbars at their full width/height so the corner got rendered,
but that interfered with the display of the horizontal scrollbar in
certain circumstances because it was too wide to scroll. This commit
provides that behavior with an absolutely positioned div with the same
dimensions as the intersection of scrollbars when both are visible.
ab1ede5fe6