Created attachment 157949 [details]
Screenshots (before - after)

SUMMARY
***
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***
The attached screenshot demonstrates the issue with a "Show Desktop" widget, but the difference gets more noticeable with bigger widgets like the system tray.

STEPS TO REPRODUCE
1. Create a vertical panel with a launcher menu, a digital clock, and a fixed-size icon-only task manager wrapped in 2 flexible spacers.
2. Try adding more widgets to the bottom end of the panel.

OBSERVED RESULT
The task manager moves higher with every new widget.

EXPECTED RESULT
The task manager remains in place, centered relative to the whole panel.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20230404
KDE Plasma Version: 5.27.3
KDE Frameworks Version: 5.104.0
Qt Version: 5.15.8
Kernel Version: 6.2.9-1-default (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION