Comment 8 for bug 1979823

Alright, I could somehow reproduce.
I don't see the wiggeling you mnention, but shrinking the sidebar is not as smooth as it's supposed to be: when grabbing the splitter and dragging it to the left, the split bar hangs at certain points, then suddenly catches up with the moving cursor. Any size in between these points can only be set by dragging to the right.

Anyway, the fix from https://github.com/mixxxdj/mixxx/pull/4808 solves this, too.
I'll port it back to 2.3