I got so fed up with plasma 5 that, even though I really didn't have enough
time if something went wrong, I installed TDE anyway this morning.
My blood pressure immediately dropped several points.
Quite a lot of configuration to do when I have time and can find instructions
on some of the necessary configuration, but at least my desktop now doesn't
behave as if it has a mind of its own: things don't disappear on me any more,
and behaviour is now consistent.
Thanks for the encouragement, everyone.