1. A computer-implemented method comprising: displaying a user-interface on a substantially transparent display of a wearable computing device, wherein: the user-interface comprises a view region and at least one content region that is located outside of the view region, the view region substantially fills a field of view of the display, and the at least one content region is not fully visible in the field of view; displaying a virtual object in the view region at a focal length along a first of sight; determining a physical distance, along the first line of sight, to a physical object; determining that the focal length is substantially equal to the physical distance to the physical object, such that the virtual object and the physical object appear substantially co-located, in a user-view through the view region, at the physical distance; and responsive to the determination, initiating a collision action between the virtual object and the physical object, wherein the collision action comprises a graphical modification of the virtual object.