Panel in hiding mode flickers when opening application menu

Submitted by Yousuf Philips @philipzae

Assigned to Xfce Bug Triage

Link to original bug (#16765)

Description

Steps:

1. add classic application menu to panel
2. set panel to always hide
3. click to open application menu
4. notice flicker in the menu

https://youtu.be/wVJVixbF5hQ (flicker appears at 00:29)

Version: Unspecified