Affected versions

• Firefox 85.0.1 (BuildId:20210204182252)
• Firefox 86.0b7 (BuildId:20210207185447
• Firefox 87.0a1 (BuildId:20210207214322)

Unaffected versions

• Firefox 78.7.1esr (BuildId:20210204185103)

Affected platforms

• Windows 10 64bit
• macOS 10.14

Unaffected platforms

• Ubuntu 20.04 64bit

Steps to reproduce

1. Launch Firefox.
2. Access several websites (just to generate some history).
3. Access the Library panel.
4. Click the “History” option.
5. Right click on any website from the “Recent History” section.
6. Mouse over slowly on the available options from the context menu.

Expected result

• No UI issues are encountered.

Actual result

• The context menu elements are flickering.

Regression Range

• This seems to be a regression:
  24:46.48 INFO: Last good revision: 9d2e5cd8c3c9c728d9d5e027fefd2fd3989b1880
  24:46.48 INFO: First bad revision: 2c022fc5b638633885787aa94f1e46a7e6713956
  24:46.48 INFO: Pushlog:
  https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=9d2e5cd8c3c9c728d9d5e027fefd2fd3989b1880&tochange=2c022fc5b638633885787aa94f1e46a7e6713956
• Possible regressor: Bug 1645773

Notes

• For further information regarding this issue please observe the attached screencast.
• macOS screencast
• Please note that, in order to reproduce this on Windows or if you don't have access to a trackpad, you need to slowly focus through the context menu elements (this issue is much easier to reproduce using trackpads).
• Please feel free to change the component if necessary.

Definitely not a GTK bug. It seems like reflowing might cause the mouse position to be resynthesized or what not, which causes the hover effect.

A very easy workaround in toolkit could be to specify backgrounds in UA or author sheets consistently.