From bug 1241720 comment 15, some UI suggestions that it seems were missed in the original device modal bug. Helen says: "We also chatted about the potential for the viewport being larger than the modal and what should happen in that situation. I think if the viewport is (for whatever reason) very large, the modal should hug the top, the viewport should blur out, and pointer events for the viewport should freeze until the device modal is closed: http://cl.ly/1w43071f0R1u"