Still waiting for my gitlab.torproject.org account to be approved. (The new walled garden approach is doing them a big favor isn’t it?) However, when I was poking around, believe it or not, I found this was already reported to Tor Project over a year ago. Man, things are much easier to find when you’re not looking for them.
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It’s much higher level than I thought. They found out it’s in JS code related to the circuit display feature in Tor Launcher. I overlooked that, because about:performance kept saying everything was fine when it wasn’t, but Tor Launcher apparently doesn’t show up there even when it’s running. According to the thread, disabling the circuit display feature in about:config gets rid of the bug. I gave it a try and it seems to work like a charm. It should be fine to disable that since the circuit display isn’t supposed to work anyway on Whonix. Maybe we could distribute a fix the same way the security level confirmation dialog box works? Just brainstorming at the moment.
extensions.torbutton.display_circuit to false