If I enter edit mode for a panel and try to select the height numbers by double-clicking them (so as to overwrite them after typing), this double click will not select the numbers, but will instead make the panel bigger.
STEPS TO REPRODUCE
1. Right-click the panel and select edit.
2. Try to double click the px height numbers.
Numbers are increased, and do not get selected.
Numbers should be selected, and should not be changed.
Operating System: Debian GNU/Linux
KDE Plasma Version: 5.23.0
KDE Frameworks Version: 5.86.0
Qt Version: 5.15.2
Kernel Version: 5.14.0-3-amd64 (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 2700 Eight-Core Processor
Memory: 15.5 GiB of RAM
Graphics Processor: AMD VEGA10
It's true that double-click doesn't select the number, but I cannot reproduce the size change. I suspect you're actually dragging by accident with the double-click (or else your mouse or touchpad has some jitter).
A possibly relevant merge request was started @ https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/364
Fixed by Noah Davis with https://invent.kde.org/frameworks/plasma-framework/-/commit/d3af00fa0be5d1d124dabc45ee1f18e32cbc26f1 in frameworks 5.88!