clicking empty overlay space while in edit mode?

If you enter edit mode, and then click some empty window space that you'd expect to let a click through to the underlying window (as it does in normal mode) we should display a warning or message or similar content to "exit edit mode".

Start by reproducing the behavior in edit mode: click empty overlay space and compare it with normal mode. Trace the overlay's click handling and edit-mode state, then consider the issue done when an appropriate warning or message tells users how to exit edit mode.

Written by the indexing model from the issue text.