Number scrubbers arrows sometimes don't show up or show up as squares

In a recent workshop, I saw 2 bits of weirdness with the number scrubber arrows. On one machine, the arrows did not show up at all, so we had to scrub a black rectangle. On another machine, the arrows showed up as squares.

Start by reproducing the number scrubber in a browser on environments where its arrows are missing or rendered as squares. Trace the scrubber's UI entry point and determine what consistent arrow rendering should look like across those environments; done means the arrows are visible and not replaced by squares.

Written by the indexing model from the issue text.