Sort of relevant to #1696
- Improves a number of layout elements at smaller viewport sizes
(specifically, letting button groups wrap onto next lines & ensure
titles can't shrink to 0 width)
- Adds "No page title" to places where there'd normally be a page title
but isn't
- Applies grid styling to page list area to fix overflow issues
- When selecting or loading with a page selected that's farther down on
the page list, the top of the page header could be scrolled away from,
and there'd be no way to scroll back because the area had `overflow:
hidden` applied
- Adds default width and height to image comparer element, so that it
displays correctly when images haven't loaded and doesn't change layout
when images load
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Co-authored-by: Henry Wilkinson <henry@wilkinson.graphics>