Left-clicking in the Preview window no longer zooms a fitted image on v1.7.0 the way it did on previous versions (including 1.6.5 still installed on another PC.)
The usual behaviour on left clicking the preview image was to zoom to full size and activate a pan tool so I could pan around the full-size image. I do that a lot.
EDIT: Ah-ha! I found the change... the Interface/Mouse settings are different. I needed to change Left Button from Move to a new setting, Zoom & Move.
Problem solved, not a bug

I'll not delete the post in case anyone else comes here to say this.