Created attachment 133953 [details]
a submenu of the radiotray-ng context menu located out of the screen.

SUMMARY

I have a default vertical panel located at the right edge of the screen. When I  right click on a widget located on the panel the widget's context menu appears snapped to the left of the panel. If the menu have submenues, sometimes these submenues appears at the right side of the panel, that is, they go to out of the screen. It may be any widget or tray icon, for example radiotray-ng. It occurs only sometimes without a pattern.

By the way, sometimes the context menu appears snapped to the right edge of the screen (which appears ugly). I think out-of-screen context menu issue doesn't occur when it is snapped to the screen edge. 

STEPS TO REPRODUCE
1. Locate a default panel at the right edge of the screen (with taskbar etc.)
2. Right click on a widget on the panel and see the context menu at the left of the panel. 
3. If the menu have an entry with a submenu (with a '>' on the right of the menu entry), hover the mouse on the entry. 

OBSERVED RESULT
The submenu, not always but often, appears at the right of the context menu, that is, it goes off-screen.

EXPECTED RESULT

The submenu must appear at the left of the context menu.
SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Manjaro KDE
(available in About System)
KDE Plasma Version: 5.20.4 (and older versions)
KDE Frameworks Version: 5.76.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Not with a panel at the left edge, since the submenus opens always to the right (and arrows '>' are pointing to the right).
I have this issue for years, with kubuntu, opensuse and manjaro.