SUMMARY
The context menu for Dolphin's left sidebar and its items appears as a floating window in the center of the current display that Dolphin is on, if Dolphin is not in focus when it is activated. This appears to only affect Dolphin, I can't reproduce it in other applications.

If Dolphin is in focus, the context menu opens as normal. It also only affects the context menu for the sidebar (the one that has "Places" and the folder list, etc).

The window will also respond to mouse movement even if the mouse is not in the "context menu window", highlighting the list elements as if the mouse is hovering over them in the window. This is the case until the mouse moves into the context menu window.

When the context menu window opens, Dolphin is not brought into focus of course, but when the window is closed (either with the window's 'X' button or by pressing the Escape key), Dolphin is then brought into focus.

Screenshot: https://i.imgur.com/WJubnU3.png

STEPS TO REPRODUCE
1. Open Dolphin
2. Unfocus it (even just clicking on the desktop behind it will work)
3. Right click on the left sidebar of Dolphin
4. The context menu will appear as a flowing window in the center of that display

OBSERVED RESULT
Dolphin's left sidebar context menu appears as a floating window when Dolphin is not in focus 

EXPECTED RESULT
The left sidebar context menu should open as a regular context menu regardless of window focus, and bring Dolphin into focus too.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.0.9-zen1-1-zen
KDE Plasma Version: 5.26.3 
KDE Frameworks Version:  5.100.0
Qt Version: 5.15.7

ADDITIONAL INFORMATION 
- Did not test X11
- I am using Plasma 5.26.3, but this also affected my laptop on 5.26.1 before I updated it
- Possibly related, resolved issue: Bug 442779