• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
Don't alt-tab during loading when using windowless mode. (Or, at least, wait until the first loading screen appears.) This is not an LI bug, but vanilla.
 
It's a mode you set in settings.txt, as opposed full screen or windowed. You might want to edit that, and set full screen.
 
It's windowed without borders (might not have the term exactly right... got it wrong, it's borderless, not windowless.)