I ran into the same issue with my laptop (3840x2160) extending to my dual monitors, which have a native resolution of 2560x1440.

What fixed it for me was to close all my open xaml files, close Visual Studio, then go to the Display Settings and set the Display Resolution on all of my monitors to the same resolution; for me that was 2560x1440.

I guess Visual Studio didn't like that I accidently had one monitor extending to 3840x2160 while the other one was at 2560x1440.