I noticed that clicking a stacked tab’s title doesn’t always make the tab fully slide out. It does get focus, but sometimes slides out just a little, with part of the tab remaining hidden behind other tabs. Clicking the title a second time does make it slide out all the way.
I was about to file a bug report for this, but then found out the in the Sandbox vault, the behaviour is as expected, with the activated tab appearing at full width.
I could try switching off my plugins one by one, but perhaps someone else already figured out who the culprit is?