SUMMARY
Not sure when exactly this has regressed (or maybe it was like this from the start), but… there are several issues with the Panel Configuration mode.

When Width (Height for vertical panels) is set to Custom:

1. Panel size guides (SliderHandle.qml) sometimes releasing the mouse drag for no reason.

When Alignment is not Center,

2. Panel size guides on the side opposite to the configured Alignment are missing without explanations. There surely is some reason for it, but it looks more like a glitch than an intention.

3. When the panel's center guide has been moved around, changing Alignment might only work on second try, or even switch to some other random alignment.

4. The whole background for size guides (the Ruler.qml component) is glitchy: its thickness jitters while dragging the handles — at least on X11 @ 150% scaling. The 1px thin perpendicular line (representing the center of the screen, I guess?) flickers are often disappears, probably for the same fractional scaling reasons.

Some other issues:

5. The centering handle is not intuitive for Left & Right alignments: what's the point of it staying at the center of the panel? It does not represent any panel's center there.

6. All these things could use some tooltips, also explaining their double-click "return to defaults" behavior.

7. When moving the center of a small panel left & right, the preview of the desktop resizes. You can briefly notice that one copy of the wallpaper leaks out as it tries to catch up with the resizing animation.

OBSERVED RESULT
bugs

EXPECTED RESULT
features

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.0
Kernel Version: 6.12.1-arch1-1 (64-bit)
Graphics Platform: X11