It seems that after using Mission Control to thumbnail all open windows, InDesign no longer allows me to click onto other application windows that I can see underneath InDesign. So If I click on any window or desktop item visible, I get caught switching to InDesign, almost as though there's an invisible window still open and becoming activated. I can be in another app, try to click to my desktop, and it'll switch over to InDesign instead.

Currently using InDesign CC v 13.0.1
macOS 10.12.6

If I quit InDesign and relaunch, it'll work again for a while. I think until I use Mission Control's window exposé again.