prathiksha first noticed this yesterday, and we've also heard reports from Mossop and vchin. So far, all of the affected users seem to be on macOS, but it's quite possible this is a cross-platform issue.

Here are the STR as I understand it:

1. In an affected Nightly, open up the AppMenu
2. Click on the Help submenu
3. Click on the "back" icon at the top of the menu
4. Repeat steps 2 and 3 until "back" stops working

ER:

The panel should always allow you to go back.

AR:

The panel stops painting at some point when going back. Interestingly, when in this state, it looks like the events are being dispatched properly to the panel that should be painted. So, for example, after clicking "back", despite the Help menu still being displayed, clicking around in that panel causes the root AppMenu items to handle the clicks.

This is somewhat intermittent, but once the bug takes hold, the panel stops painting from there forward. I haven't personally seen this bug yet, so somebody who has maybe can provide additional context or corrected STR.

prathiksha found this regression range:

7:55.19 INFO: Last good revision: ae11d50c40266fd3413ee79ef86e6ccb1dbbed9c
7:55.19 INFO: First bad revision: e505fffd43c3663a2ed608907ed17830cc526ec9
7:55.19 INFO: Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=ae11d50c40266fd3413ee79ef86e6ccb1dbbed9c&tochange=e505fffd43c3663a2ed608907ed17830cc526ec9

FWIW, this happens for me systematically, not just intermittently, on FF92.0b2 macOS 11.5. It is 100% reproducible with every "non-standard" pop-up menu, i.e. both the "hamburger" menu and the toolbar bookmark pop-ups. I only need to display a given menu twice (it's always twice, never just once or three times) before it stops redrawing itself when hovering over it.

[Tracking Requested - why for this release]: Serious regression

I can get this to reproduce reliably with themes. I imagine it's the same bug.

STR:

1. Apply the Dark theme.
2. Open the app menu.
3. Apply the Light theme.

ER:
The light app menu is shown.

AR:
The dark app menu is shown.

The patch in bug 1724848 has landed. It will probably fix this.

The fix should be in the latest Nightly. harry or Martin, are you able to confirm that this is fixed?

I can no longer reproduce using my STR.

Good enough for me! I'm going to dupe this over then to 1724848, since I guess they were ultimately the same bug.

Yes, it appears to be fixed in the nightly build on my end.