Ah! I've just made a discovery! If you Quit X1 completely, and then restart it, it starts maximised, and the problem doesn't reoccur.
It seems to only happen on the initial load, after start-up of the PC, if you have opted to "Start X1 Minimised". So I think I have solved it by unticking the "Start X1 Minimised" box in Options.
It seems to only happen on the initial load, after start-up of the PC, if you have opted to "Start X1 Minimised". So I think I have solved it by unticking the "Start X1 Minimised" box in Options.