An application running on a computing platform that employs three-dimensional (3D) modeling is extended using a virtual viewport into which 3D holograms are rendered by a mixed-reality head mounted display (HMD) device. The HMD device user can position the viewport to be rendered next to a real world 2D monitor and use it as a natural extension of the 3D modeling application. For example, the user can interact with modeled objects in mixed-reality and move objects between the monitor and the viewport. The 3D modeling application and HMD device are configured to exchange scene data for modeled objects (such as geometry, lighting, rotation, scale) and user interface parameters (such as mouse and keyboard inputs). The HMD device implements head tracking to determine where the user is looking so that user inputs are appropriately directed to the monitor or viewport.