Found in

• 107.0a1 (20221012213343)

Affected versions

• 107.0a1 (20221012213343)
• 106.0
• 105.0.3
• 102.4.0esr

Tested platforms

• Affected platforms: Windows 10x64
• Unaffected platforms: macOS 10.15, Ubuntu 20.04

Steps to reproduce

1. Open about:preferences in the General section and press the keyboard Tab key until Size or Default zoom dropdowns are selected
2. Click on the dropdown to open it and observe the dropdown.

Expected result

• Dropdown menu is opened as expected.

Actual result

• Dropdown menu width is increased when open.

Regression range

• Last good revision: 64212702d9f0d6410362b209dcf0348cb3acb452 (2021-09-11)
• First bad revision: 561ed5ccdd2400934cb05151dbf8dbe105bfc963 (2021-09-12)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=64212702d9f0d6410362b209dcf0348cb3acb452&tochange=561ed5ccdd2400934cb05151dbf8dbe105bfc963
  Unfortunately, I don't know for sure if bug 1729854 is the regressor.

Additional notes

• Attached a screen recording of the issue.
• This happens while hovering over default search engines while the dropdown menu is selected.
• The issue does not reproduce if the dropdown is not keyboard-selected before opening.