STEPS TO REPRODUCE
1. log into your plasma session
2. add "volume widget" or "battery and brightness widget" to your desktop (not the panel)
3. try to change the volume or the brightness 1% up or down

OBSERVED RESULT
you will be switched into "edit layout" mode in 99% cases because you are holding a mouse on a widget (even worse when use touchscreen). 

EXPECTED RESULT
you can hold mouse on a volume slider because you are trying to change volume but not edit the widget size.
edit mode should start only when you hold an empty area of widget imho. when you hold a slider, that should not start edit mode. 


SOFTWARE/OS VERSIONS

Linux/KDE Plasma: Opensuse 15.2 
KDE Plasma Version: 5.18 from opensuse repos

Linux/KDE Plasma: Opensuse 15.2 
KDE Plasma Version: 5.21 from KDE repos for zypper

Linux/KDE Plasma: debian stretch (9) 
KDE Plasma Version: 5.12

Linux/KDE Plasma: gentoo 
KDE Plasma Version: 5.22.5 built with -march=native -O2, tried both clang and gcc

btw in debian 9 there was a "lock/unlock widgets" button. it is that old version where widgets do not enter edit mode when you hold them so there's no bug.

ADDITIONAL INFORMATION
sorry for my english, let me know if something is not clear