Screen header moving (up and down, left or right)

Hello, having issues with my screen moving or jumping around. I didn't change anything so not sure why it's doing this. Everytime I scroll up or down the header moves... This is a screen shot, which came out fine, of course. However I'm looking at a half cut off apple logo, firefox and so on...

How do i get this back to normal? It's movement is annoying and can't figure this out.

I'm using a 27' iMac and using Big Sur. Only noticed this when I upgraded to BS.

Any help would be appreciated.