Hi, and thanks for the hints. These were discussed over at arduino.cc a while back, and I had some small measure of success with them, but it was very messy to get this working with the apps that needed them (only). And, ultimately, it broke savagely with problem #2 -- when I move between displays with drastically different DPI's. I don't think that second problem is totally intractable ... but I do think it's a long way of being fixed (on any platform).

EDIT: Oh, and yes, the 'scaling support' feature was added into the version that I was trialling of arduino IDE a while back (I forget the #'s). It was meant to have been resolved, but I noted on the various forums that lots of people (including myself) couldn't get it to play nice. Investigations were on-going whether it was a KDE, xorg/wayland, nvidia, phase of the moon, or some other related problem.