Affected versions

• Firefox 84.0b4
• Firefox 83.0
• Latest Nightly 85.0a1

Affected platforms

• Windows 10 64bit
• macOS 11 (harder to reproduce for some reason)
• Ubuntu 18.04

Steps to reproduce

1. Start Firefox
2. Enable Bookmark Toolbar (ctrl/cmd+shift+B)
3. Right click any bookmarks and hit Properties
4. Click the dropdown arrow from Tags twice (expand/collapse)

&

1. Start Firefox
2. Open Library (ctrl/cmd+shift+O)
3. Right click an entry from History and select New Bookmark
4. Expand and Collapse Folder or Tags menu. (repeat if the issue is not reproducible immediately)

&

1. Start Firefox
2. Open Library (ctrl/cmd+shift+O)
3. Right click an entry from All Bookmarks and select New Bookmark
4. Expand and Collapse Folder or Tags menu. (repeat if the issue is not reproducible immediately)

Expected result

• The panel goes back to its initial size after collapsing Tags or other hidden sections.

Actual result

• Extra space is visible after collapsing the hidden menus.

Regression range

• This is not a recent regression, I also reproduced this in Firefox 60 (though it is harder to reproduce). I can search for a regression if necessary.

Additional notes

• This also happens when
• I have a feeling this is already logged somewhere, I could only find bug 940288 which is a very old one.

Suggested severity

• Keeping in mind that this is an old issue I would suggest a low severity as well, S4.

it seems intermittent, sometimes it shrinks, sometimes not, probably depending on when the calculation happens