Basically this, except much faster:
On latest version of Discourse, using Chrome. Only non-default plugin is Spoiler Alert.
Repro steps somewhat indeterminate - it seems to happen when I have a read PM notification, I’m not sure if there are other times when it happens. Only shows up with the full screen width version of the notification menu (not the “slide-in” version, I think is what it’s called).
Looks like scroll bar is triggering resize event. Not sure if this is enough for @eviltrout to reproduce.
Yeah, sorry for the sparse detail, I haven’t been able to get it to trigger much. It went away after getting further notifications (an @ and a reply), so far it’s 2 for 2 on needing a read PM notification at the top, as odd as that seems to me.
The gif is changing so fast it’s hard to tell, but I don’t think it is because of the type(s) of Notification, but
That’s my guess.
The only reason I can think of that might be responsible for the rapid scrollbar ↔ no scrollbar is that the “Log Out” isn’t being considered as part of the scrollable area when there is a scrollbar resulting in a “fits ↔ doesn’t fit” thing happening.
I really can’t get this to happen on a mac even though I am pretty sure I know what is happening code wise. I will try again on Windows soon and hopefully the different scrollbar shape will make it easier to reproduce and fix.
Update: I spent a while this morning testing various other browsers on windows and still can’t reproduce it 
Is this site running the latest version of Discourse? I did some tweaking to that menu and I am wondering if the tweak caused it, or if perhaps it fixed it as I can’t reproduce it.
It was on latest, I’m a few commits behind now. I’ll see if I can get it to happen again today when I’m at latest again, will update when I have more info!
I haven’t been able to reproduce this behavior since updating to latest, so… fixed, apparently?
Great! I’ll close this for now. If it happens again we can revisit!