SUMMARY

In my setup, the panel is at the bottom of the screen (as usual) and autohides.
Each time I show the panel (by moving the mouse cursor at the bottom of the screen), errors are generated in .xsession-errors file :

qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 55586, resource id: 25175127, major code: 3 (GetWindowAttributes), minor code: 0

The sequence and resource id always change.

Please note:
- the "window" key of my keyboard acts as a shortcut to display the panel. If I use that shortcut, then no error is produced !
- when I bring the mouse cursor close the panel, a blue line (visual hint) is displayed. The error doesn't occur at that moment. It occurs when the cursor actually triggers the display of the panel.


STEPS TO REPRODUCE
1. Let the panel hide itself
2. Display the panel by moving the mouse cursor on it
3. 

OBSERVED RESULT

Panel displayed. Errors produced.

EXPECTED RESULT

Panel displayed. Errors produced.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian Bullseye (stable); Nvidia gpu.
(available in About System)
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78
Qt Version: 5.15.2

ADDITIONAL INFORMATION

I've read the other bug reports suggested by the bug tracker (337690, 390403). They seem to correspond. However, if Wt warns of an issue, then that issue my be caught before it happens ? 

I've also checked the Qt documentation about a QXcbConnection class, but nothing  was to be found :-(