Photoshop screen far too big for imac display

I'm hoping someone can help me with this as it's driving me crazy!  In Standard Screen Mode in PS the bottom of the screen goes way out of my display with a large gap at the top of any document, which means I can never see the whole of an image on my screen.  The scrolling bar only appears when I'm really zoomed in. You can see the issue on my screen shot.

This never used to be an issue and I've tried looking at various settings and pereferences but can't find anything to fix it...