Painter 2021 window resizes itself?

I'm using Painter 2021 on a PC with Win10.

With Painter open and maximized on my screen, if I open an image and maximize it, then open a second image, the whole Painter window shrinks, (with both image windows still contained in it, but it leaves my palettes scattered outside of it, all over my desktop.

It doesn't do this if I open an image and don't maximize it, and then open a 2nd image. 

But it's really annoying. I don't know if there is some setting I should change, is it a Painter thing or a Windows thing or ???

Here's a screen shot: