Don’t think this has been mentioned before and I can reliably recreate this bug:
I have set the left and right page margins to zero on a layout so that I can achieve some full width graphics on the title page, and adjusted the frame margins in the master pages to compensate as discussed numerous times on this forum. That’s all working fine BUT now in engrave mode staff spacing I am unable to click the boxes between staves for manual mm input. Dragging/Alt+arrowing the handles still works, and problem goes away if I put the layout page margins back to default.