I attempted to preserve the "don't draw unless different" by ditching rounding for more
precise display_span, but that didn't work. An alternative solution would be
to draw on adjustment change if there's text, since then we need to redraw
regardless of slider position, but it seemed weird even just with respect to
the slider, so I opted for this, which really definitely redraws when the
adjustment changes, period.
If this proves to be a performance issue we'll have to figure that out.
