Affected versions

• 85.0a1(20201214091023)
• 84.0 (20201211215739)
• 83 (20201112153044)
• 78.6.0esr (20201207224150)

Affected platforms

• Windows 10x64
• Ubuntu 20.04
• macOS 10.15

Steps to reproduce

1. Open Firefox with a new profile and resize it horizontally until the More tools button is displayed.
2. Restart Firefox.

Expected result

• Hamburger menu button is displayed as expected.

Actual result

• Hamburger menu button is shifted.

Regression range

• Last good revision: 19704452bd548d0c36d601d609cfcfe2c3e0caa2 (2019-09-19)
• First bad revision: c221a75fbf2957b6254fedb96436b5efbf0fd147 (2019-09-20)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=19704452bd548d0c36d601d609cfcfe2c3e0caa2&tochange=c221a75fbf2957b6254fedb96436b5efbf0fd147

Notes

• Attached a screen recording.
• I originally encountered a similar issue while updating Nightly with a small window. The difference was that the Open menu button was no longer displayed after the restart to update and the More tools button was cut off. I can no longer replicate the original issue after I closed Firefox, neither on a new profile but I have a screen recording with the issue reproducing while restarting firefox with a small window: link

Suggested Severity: S4

I don't know if we can (easily) fix this without re-introducing overflow:hidden (perhaps temporarily). Dão, do you have ideas off-hand?

Hey, we haven't heard from you on this Dao - just pinging it to surface again.

I'm also going to set Severity / Priority - this is a bit of flicker and a bit distracting when opening a window with a small width dimension, but is certainly non-catastrophic.