Created attachment 164630 [details]
video showing default panel widgets jump around as applications close

SUMMARY
When creating a new default panel (and I'm assuming the same for a new user), the new panel has:
- Application Launcher
- Desktop Pager
- Icon Only Task Manager
- Margin Separator
- System Tray
- Clock
- Show Desktop

Its default configuration is to be Centered and Fill Width.

Because the it fills the width of the window, and the Icon Only Task Manager is very narrow, especially if there aren't many apps running, then on a wide screen, the other widgets will get evenly spaced on the very wide and empty panel, until there are some applications running - but when applications launch and get removed, the widgets move around.

This can be also reproduced with the Task Manager (not "Icons Only"). See attached video.

STEPS TO REPRODUCE
1. Remove all panels and create a default panel.
2. Launch some applications.
3. Close some applications.

OBSERVED RESULT
The fixed widgets on the panel move around when applications open and close.

EXPECTED RESULT
The space between the last task in the task manager list and the system tray or its margin separator should shrink and expand as needed, while keeping everything else in the same location.

SOFTWARE/OS VERSIONS
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.91.90
KDE Frameworks Version: 5.248.0
Qt Version: 6.6.1
Kernel Version: 6.2.0-39-generic (64-bit)
Graphics Platform: offscreen
Processors: 20 × 12th Gen Intel® Core™ i7-12700H
Memory: 31.0 GiB of RAM
Graphics Processor: Mesa Intel® Graphics

ADDITIONAL INFORMATION
The workaround is quite simple - go into edit more, click "Add Spacer" in the Panel Settings dialog, and drag it between the task manager and the margin separator. I just shouldn't have needed to do that.