Described is a technology by which an application program such as a gadget may communicate with a sidebar program to output content for consumption by an auxiliary display device. The gadget may communicate with the sidebar program via sidebar APIs, and in turn the sidebar program communicates with an auxiliary display device platform via auxiliary display device APIs. Among its capabilities, the sidebar program may install gadgets on a host computer system, and upon detecting that the host computer system is configured to operate with an auxiliary display device, may install the gadget such that the gadget is configured to work with an auxiliary display device. The installation may be accomplished by having the sidebar program write information to a system registry.