* Draw the frame/time number box over the scrollbar instead of above it,
to reduce the clutter/clashes with markers.
* Draw the box centered around the line instead of off to one side,
making it clearer that the frame shown is the one being affected.
* Make the box larger than the scrollbar + use white text to make it
stand out from the neighbouring frame numbers (otherwise, it's easy
to misread that it's just another one of those)
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