1. A mixed reality computing device, comprising an image sensor; a display device; a storage device comprising instructions; and a processor configured to execute the instructions to receive, from the image sensor, a first physical environment image during a first use session; store the first physical environment image; render a three-dimensional virtual model; form a mixed reality thumbnail image by compositing a view of the three-dimensional virtual model and the first physical environment image; display, on the display device, the mixed reality thumbnail image; save the three-dimensional virtual model and the mixed reality thumbnail image in a save file; receive a user input selecting the mixed reality thumbnail image, and in response open the three-dimensional virtual model for editing during a second use session; receive a user input updating the three-dimensional virtual model during the second use session to form an updated three-dimensional virtual model; render the updated three-dimensional virtual model; receive, from the image sensor, a second physical environment image during the second use session; update the mixed reality thumbnail image by compositing a view of the updated three-dimensional virtual model and the first physical environment image from the first use session to thereby provide continuity of an overall appearance of the mixed reality thumbnail image between the first use session and the second use session; save the updated three-dimensional virtual model and the mixed reality thumbnail image in the save file; and display, on the display device, the mixed reality thumbnail image including the updated three-dimensional virtual model composited with the first physical environment image.