This enables subscribers to detect not just that stylesheets have
changed, but specifically how they have changed. This is used by the
React editor component to only refresh scrollbars when a stylesheet
that actually contains selectors for scrollbar elements is added or
removed.
A 0 measurement indicates that overlay scrollbars are enabled, so we
just fall back to 15px in that case so the user can hover directly over
the scrollbar to scroll.
We measure the scrollbar-corner node when there's a stylesheet change,
but Chromium won't apply the new style if it was already visible before
the change. This commit hides and shows it before measuring so we get
accurate values.
Previously, dummy scrollbars were always 15px wide/tall. This caused
them to obscure the ability to click for the entire 15px region, even if
the actual scrollbar was styled to be much thinner. Now we explicitly
measure the size of scrollbars on mount and when the stylesheets change
and set the height/width explicitly.
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.
This entailed quite a few changes to dial in scrollbars. The scrollbars
are now adjusted in size to account for the width of the opposite
scrollbar. If the width or height are not explicitly constrained and we
are scrollable in the opposite direction that is constrained, we account
for the width of the opposite scrollbar in assigning a natural height
or width based on the content.
Because the scrollbar now spans the entire editor but the scrollable
area does not include the gutter, we need to add the current width of
the gutter to the scroll width of the horizontal scrollbar to allow
it to scroll to the end of the longest lines.
We set overflow to hidden in the opposite scroll direction only if we
can't actually scroll in that direction, causing the white square where
neither scrollbar overlaps to appear at the lower right corner.
