Menu options need extra click to activate

Possibly a regression related to QTBUG-31937 and/or QTBUG-54773.

When selecting an application menu with the LMB and, with the LMB still pressed, moving to the desired menu option and releasing the LMB, this option won't be triggered.

The menu stays open, another click with the LMB is necessary to trigger the option.

The same behaviour can be seen in the desktop context menu,  using the RMB to open the context menu and selecting the menu option.

Steps to reproduce:

1. Click application menu with the LMB (keep LMB pressed)
2. Move mouse cursor to desired menu option
3. Release LMB
4. Nothing happens

Affected applications:

Apparently all Qt applications (e.g. kate, konsole, kshisen, desktop context menu etc.). Gtk applications used in the same KDE/Plasma environment work as expected.

This behaviour was introduced after Qt was upgraded from 5.14.2 to 5.15.0. When booting the live system used for installation, all Qt applications work as expected.

On another machine running Debian testing with KDE/Plasma 5.17.5 and Qt 5.12.5, all applications work as expected.

Cheers