debug vertical position of slider dots

[meganrm]

          fyi: this appeared a bit misaligned when I tested (in Firefox and Chrome):

I don't know that I have any suggestions for a better way to do alignment though... maybe the progress bar segments could get an inner container for just them and the ::before could be vertically centered within that with something like margin: auto 0; top: 0; bottom: 0;? This also may be within the range of "good enough" anyways, up to you. I don't envy you having to deal with vertical centering here.

Originally posted by @frasercl in #50 (comment)