Created attachment 149261 [details]
comparison of digital clock applet menu when compositor on/off, when panel are smaller/bigger

SUMMARY

***
when digital clock are placed in the middle of the screen inside panel, if you click it will show a menu, but the menu are not centered when compositor is on, instead display are attached to the left end of the panel, this however are not happen when you suspend compositing.
***

STEPS TO REPRODUCE
1. create/configure panel with panel alignment : centered
2. put digital clock inside panel, and add two spacer between
3. set the panel's length to be smaller than the digital clock applet menu
4. click the digital clock applet to show the menu
5. compare with when compositor is off(suspended) by shortcut key "Shift+Alt+F12"(suspend compositing) 

OBSERVED RESULT
when panel length smaller than digital clock menu
if compositor is on, it will attached to the left end of panel istead of centered
if compositor is off, then it will centered as when compositor is on and panel's length bigger than digital clock menu 

EXPECTED RESULT
digital clock applet menu are centered,
whether panel length shorter or larger than  digital clock applet menu,
whether compositor on/off


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Archlinux Packages
(available in About System)
KDE Plasma Version: 5.24.5
KDE Frameworks Version: 5.94.0
Qt Version: 5.15.4

ADDITIONAL INFORMATION
sorry for the lack of grammar and informality in the report, thank you from indonesia