SUMMARY
Since the upgrade to Plasma 5.23.0 (KF 5.86.0, Qt 5.15.2), using the menu of tray icons causes chaos with the input methods.
Possibly it is only mouse input, but that's hard to say as it literally causes chaos.

The first time I click, I can use the menu of some tray icon (here mostly cantata and xpra) as usual. The second or third time, the menu does not work, but afterward moving the mouse causes a lot of random clicks in random windows, until I change to the Linux console, run "killall plasmashell", switch back and then restart plasmashell.

Things that may be relevant:
- I use a vertical panel on the left side of the screen and the tray area is at bottom
- I use focus follows the mouse (which probably explains why it seems like random windows are target of the random clicks)

I do not know where to start debugging the issue. The only thing I can say is, that it started with new plasma/kf packages in Debian testing and since then constantly happens.
If there were major changes in plasma/panels/systray between the previous version and 5.23.0, it would be help to know where to start looking for what may cause the issue.