A head-mounted display system, comprising: a head-mounted display configured to display one or more virtual objects in a physical environment, the one or more virtual objects including an application program virtual object associated with an application program and displayed at a first location in the physical environment; an inward-facing optical sensor configured to detect a gaze direction of a user of the head-mounted display system; and a processor configured to determine a field of view of the user based on the detected gaze direction of the user, characterized in that the processor is further configured to: receive a three-dimensional push notification at the application program via an application program interface, API, of one or more APIs included in a volumetric operating system, the three-dimensional push notification including location data including a second location relative to the first location in the physical environment; generate a three-dimensional push notification virtual object based on the three-dimensional push notification; determine whether or not the second location of the three-dimensional push notification virtual object is inside the determined field of view; responsive to determining that the second location of the three-dimensional push notification virtual object is inside the determined field of view of the user, output the three-dimensional push notification virtual object for display on the head-mounted display at the second location relative to the first location in the physical environment; and responsive to determining that the second location of the three-dimensional push notification virtual object is outside the determined field of view of the user, display an animated icon included in the three-dimensional push notification virtual object and pointing toward the first location of the application program virtual object within the determined field of view of the user for directing the user's gaze toward both the application program virtual object and the rest of the three-dimensional push notification virtual object. a head-mounted display configured to display one or more virtual objects in a physical environment, the one or more virtual objects including an application program virtual object associated with an application program and displayed at a first location in the physical environment; an inward-facing optical sensor configured to detect a gaze direction of a user of the head-mounted display system; and a processor configured to determine a field of view of the user based on the detected gaze direction of the user, characterized in that the processor is further configured to: receive a three-dimensional push notification at the application program via an application program interface, API, of one or more APIs included in a volumetric operating system, the three-dimensional push notification including location data including a second location relative to the first location in the physical environment; generate a three-dimensional push notification virtual object based on the three-dimensional push notification; determine whether or not the second location of the three-dimensional push notification virtual object is inside the determined field of view; responsive to determining that the second location of the three-dimensional push notification virtual object is inside the determined field of view of the user, output the three-dimensional push notification virtual object for display on the head-mounted display at the second location relative to the first location in the physical environment; and responsive to determining that the second location of the three-dimensional push notification virtual object is outside the determined field of view of the user, display an animated icon included in the three-dimensional push notification virtual object and pointing toward the first location of the application program virtual object within the determined field of view of the user for directing the user's gaze toward both the application program virtual object and the rest of the three-dimensional push notification virtual object. receive a three-dimensional push notification at the application program via an application program interface, API, of one or more APIs included in a volumetric operating system, the three-dimensional push notification including location data including a second location relative to the first location in the physical environment; generate a three-dimensional push notification virtual object based on the three-dimensional push notification; determine whether or not the second location of the three-dimensional push notification virtual object is inside the determined field of view; responsive to determining that the second location of the three-dimensional push notification virtual object is inside the determined field of view of the user, output the three-dimensional push notification virtual object for display on the head-mounted display at the second location relative to the first location in the physical environment; and responsive to determining that the second location of the three-dimensional push notification virtual object is outside the determined field of view of the user, display an animated icon included in the three-dimensional push notification virtual object and pointing toward the first location of the application program virtual object within the determined field of view of the user for directing the user's gaze toward both the application program virtual object and the rest of the three-dimensional push notification virtual object.