The dolphin browzer has this feature that allows you to slide from the left, or from the right, and then see bookmarks (left), or addons (right).
How can I program my web app to shift over 500px to the right and stay there unless the user tries sliding the window or pinching to make smaller?

The purpose of what I am doing is to allow a web form that is wider then the screen, but also have it snap either to the right half of the form, or the left half.

Thanks