Affected versions

• Firefox 89.0a1

Affected platforms

• Ubuntu 20.04

Steps to reproduce

1. Launch Firefox
2. Enter customize mode and add some icons that has submenus to the overflow menu (bookmarks menu button for example)
3. Save the changes and open the overflow menu and click on bookmarks menu to open it and close it several times

Expected result

• The menu has a smooth transition

Actual result

• The menu shakes while it is opened and closed

Regression range

• Will see for a regression.

Additional notes

• The issue can be seen in the attachment
• The issue is not reproducible on 88.0b9 or 88.0.
• The issue is reproducible on 89.0a1 with Proton prefs off.

Can you please try to find a regression window ASAP while this is still a nightly regression?

Sure thing. Mozregression seems to be pinpointing between 2020-07-22 and 2020-07-23.
This is the pushlog between the two main branches: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=138e7b575614cbfc1e45576a15825f51cb6e6614&tochange=38eb7e99885965254de8ab4540713961780ff1a5

By going further within autoland builds, mozregression pinpoints to Bug 1653443, from pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=bfc0213871d59a5ed81e71c16378f3a656cfce12&tochange=5df699bb72b1c9f9a18094e8c2e1df42bfd7e39b

I think I've seen similar issues with some other panels, but if it doesn't affect beta, and was reproducible without proton on nightly, then we might be OK for proton...