Stuff in Viewport is Masked Off

Somehow I have accidentally hit a combination of hot keys that resulted in everything outside of a rectangle (as seen from the camera), being hidden. This persists in other views - everything outside the cone of the camera view is hidden. I meant to do a border render, but when I defined my border everything outside of it disappeared. How do I reveal my full scene again? And what combination of hotkeys did I hit to get this function?

Sounds like alt+b (press alt+b again to deactive) or the passepartout. But it doesn’t sound very much like the passepartout.

alt-b it was. Thanks!