UPDATE: This seems to be a problem with 5.1 in general. Manipulating or even selecting the window in any way will cause this error, at least on Windows. After determining that it was an issue with the engine’s code and not mine, I created a brand new project in 5.1 and got the same error. I tried updating ever driver I could think of and restarted my system, to no avail. Finally I completely uninstalled and reinstalled 5.1, and am getting the same error. Looks like I’ll be rolling back to 5.0.3.