Fix several visual issues with find result highlighting:

- Reduce width of scrollbar tickmarks to match new, narrower custom-themed scrollbars from #6305 (should also look better on Mac, where the tickmarks were already too wide)
- Fix bug where showing/hiding a bottom panel didn't update the positioning of scrollbar tickmarks
- Results that span multiple tokens showed little orange dots in the text highlight between tokens (due to repeated corner rounding). There are still faint lines between tokens (due to box-shadow), but they're much fainter.