A mixed reality system is disclosed that displays 3D content in a container referred to as a “prism,” and also displays a related content item, such as an interactive menu for interacting with the 3D content. To maintain the content item accessible to the user, the system repositions the content item relative to the prism as the user moves relative to the prism. For example, in the context of an annotation toolbar, the system may move the toolbar from one surface of the prism to another in response to the user walking around the prism, entering the prism, or changing a head pose in the prism. In some embodiments, the content item is displayed in a reserved region in which the 3D content is not displayable, such that the 3D content does not interfere with the user's ability to view and access the content item.