Let me preface this by saying that this seems like a tough bug to track down, because the behavior seems to be performance-related. I can reproduce this infrequently under normal conditions, but I can reproduce it consistently with the profiler turned on.

STR:

1. Install the Tree Style Tabs extension (here)
2. Open several tabs
3. Control-click a link so that it opens in a new tab
4. Optionally, move the mouse around over the Tree Style Tabs side bar (this seems to exacerbate the problem).

Actual Results:
Some tabs in the tree have this weird flickering graphical glitch while the new tab is loading
Expected Results:
No graphical glitches

It seems like the specific condition necessary to trigger this is that a tab is loading but that tab is not the currently selected tab. Oddly, the tabs that flicker seem to be random; they are not necessarily the currently selected tab, nor the loading one. The situation is especially bad if multiple tabs are loading. When I opened my browser this morning and my pinned tabs all started loading at once, it was like a blizzard of graphical glitches.

I attached a screenshot of the problem occurring. It doesn't capture how "flickery" the tab was, but you can see that it doesn't look right. The text on that tab is supposed to read "checkUpdateManager".

I never saw this bug before today. But today I started running the profiler which, as I mentioned, seems to exacerbate the problem.

Environment:
Nightly 83.0a1 (2020-09-29) (64-bit)
Windows 10

Hmm. Actually, it appears to not be a requirement that the loading tab is not the current tab. I've been seeing the issue even when the only tab loading is the current tab. My mistake.

It should be fixed with latest Nightly.

Ah, right you are! I updated and it looks like it's working fine now. Thanks!