1. A method comprising: by a computing device, detecting a touch event on a side-mounted touchpad of the computing device during display of a current user interface of an application executing on the computing device, wherein the application comprises a navigation hierarchy that comprises a home user-interface layer, a plurality of sub user-interface layers, and a plurality of user-interface branches; by the computing device, determining a particular sub user-interface layer and user-interface branch that the current user interface corresponds to; by the computing device, determining a type of the touch event; by the computing device, when the type of the touch event is a forward touch event and there is a next sub user-interface layer in the particular user-interface branch of the current user interface, navigating to the next user-interface layer in the particular user-interface branch of the current user interface; and by the computing device, when the type of the touch event is a backward touch event and there is a previous sub user-interface layer in the particular user-interface branch of the current user interface, navigating to the previous user-interface layer in the particular user-interface branch of the current user interface.