>I'm on Windows 10 and I just tried my copies of FireFox, ThunderBird, Notepad++ and a couple of games I happen to have running, and none of them will drag once they are maximised.

Just had to try this and it works - picked a random window, doubleclicked on the titlebar to maximize it, then tried to drag. It unmaximized on the spot, and went where I dragged it.

Linux Mint 21.2 Victoria. Sorry about your OS (gd&r).