Grr.
I have been struggling with schema compare and editing anything SQL related in Visual Studio for a couple of weeks now. Cursor constantly flickering, can't click on anything, and actually running the compare results in Visual Studio disappearing in puff of smoke.
I'd put it down to an update, but then this week's update didn't fix it.
Turns out it is a "feature" that Microsoft has been refusing to fix for over a year now, where none of the SQL stuff in VS works properly on a 4k external monitor. And of course I only started trying to use that stuff on my 4k monitor since I have been working from home.
Dragging VS across to the laptop screen solves the problem (and is the Microsoft fix), but I think I now need to visit the optician to get new glasses to see WTF is going on.