User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.149 Safari/537.36

Steps to reproduce:

I used to use a community-made dark theme for Thunderbird, but since a recent Thunderbird release basically made all themes invalid or useless, I'm just on the default theme on macOS with dark mode enabled. Sometimes when the Thunderbird window is not in focus, often after a new email arrives, the scrollbar thumb on the main message window (tree#threadTree .tree-rows scrollbar) starts flickering constantly between active and inactive states. This continues until focus is restored to the window. It's not always reproducible. I will try to get a video the next time it happens.

Actual results:

The scrollbar flickers constantly.

Expected results:

The scrollbar does not flicker.

Caught it on video. This flickering is actually relatively less-flickery; usually the contrast change is greater as it's flickering.

Caught a better video. This started happening while my focus was on another window, with no obvious recent changes. But it also sometimes happens while Thunderbird is in focus as you can see in the previous recording.

Eckard, have you seen this?

I consistently see this in account settings on Macbook Air using beta. Unclear when it started. (I have webrender enabled)

I've never seen this issue in TB 68.* running macOS 10.14.6 on an iMac Retina 4k

Gus, is this something you see?

ice,, does this still reproduce for you?