If your mouse input is totally busted in Marathon right now, I may have the fix

My first minutes with the Marathon server slam were less than ideal. Immediately upon seeing a menu, my mouse cursor began to stutter, menus opened and closed randomly on their own, and basic inventory management was impossible without switching to controller.

Apparently I'm not the only one, because Bungie has issued a statement on mouse input issues both on social channels and in-game: the team is working on a fix, but their best advice right now is basically turn off everything that could possibly mess with mouse input.