Created attachment 136030 [details]
Plasma 5.21 global menu under Wayland: in Single Button mode, for Kate, showing submenu opened with window decorations, and Kate lost focus to Plasma

SUMMARY
In the Wayland session, When the Global Menu applet is configured to show a single button instead of the whole menu, the top-level menu will open fine, but opening any submenu will cause the application to lose focus and open a regular window, with decorations, showing an unrelated Plasma menu.

STEPS TO REPRODUCE
1. Put Global Menu applet in panel, configure to show a single button instead of the whole menu
2. Open a KDE application (e.g. Kate or Dolphin)
3. Click on global menu button, the menu opens
4. Hover mouse over any entry that opens a submenu

OBSERVED RESULT
The application loses focus, in place of the regular submenu a window with regular window decorations and an unrelated Plasma menu appears (see screenshot) 

EXPECTED RESULT
The regular submenu should open as usual.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20210220
KDE Plasma Version: 5.21.0
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2
Kernel Version: 5.10.16-1-default
OS Type: 64-bit
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i7-8550U CPU @ 1.80GHz
Memory: 15.5 GiB of RAM
Graphics Processor: Mesa DRI Intel® UHD Graphics 620

ADDITIONAL INFORMATION
When Global Menu is configured to show the full menu instead of a single button, all submenus work just fine.