[Build 263] Mouse Clicks not registering

I'm getting a game-breaking bug with build 263 where mouse clicks stop registering after entering 1-2 menus. It doesn't seem to matter whether it's options or hte server browser, but the game consistently stops registering any clicks after I open up one menu, sometimes 2 if it's still registering clicks after I enter one. This is a default install. I've verified game cache and removed the NaturalSelection2 folder in AppData. Any ideas?

I'm using Windows 8.1 if that's relevant at all.