Enable scrolling when mouse is in the corner "margin" around the edge of windows.

[DavidPD]

I often "throw" my mouse to the edge of the screen when I'm trying to scroll through a webpage or article, but with Zen when my mouse sits right on the edge of the window the webpage doesn't scroll.

Since most other browsers I use don't have this extra margin around the site I haven't noticed this being an issue in Safari or Firefox. This also happens in both maximized or actual full screen in Zen, at least on Mac.

I was torn on whether this should be a feature request or a bug so let me know if this should be an issue instead!