The logic for handling the active window when it is a transient, added in
https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/509, had
a bug (fixed in 67003f740888fa9d1bcee60cd28ce9a862d2452a).
This reworks the implementation and actually makes the original feature work
(a signal also wasn't properly handled).
BUG: 438222
FIXED-IN: 5.22.1