@leocg Did you get around to testing it with the posted video? It seems to start happening after the chat fills up a bit. Maybe a minute or two in.
I have tried it with no extensions, and with browser.js enabled and disabled. I have disabled the "Opening animation on start page" flag and enabled the "Show memory usage in hovercards." flag, but otherwise it's default.
I also tried disabling Aria, but I don't think my custom settings should have much effect on the issue. The main thing is that this was not an issue a few browser versions back on the same machine and browser setup, but has persisted a month or two now without being fixed. It makes it impossible to have chat open for more than a minute.