This is pretty easy to replicate, just select New Editor Window for Play Mode, and open the Editor preferences, check game viewport settings as below:
every time you click play, width will increase by 2 pixels, and height by 8 pixels.
if you check “always center first viewport window to screen” it will stop happening, but then the window will only open on your primary screen, and I prefer it on my secondary screen.
this is very annoying. is it a known bug?