In general, one aspect of the subject matter described in this specification can be embodied in a method for integrating separate software applications in an application execution environment, where the method includes providing a hierarchical structure of nodes corresponding to user interface objects representing elements of a user interface of a computer, wherein the hierarchical structure governs propagation of user interface events among the user interface objects; the providing includes enabling separate software applications to concurrently create nodes in the hierarchical structure and share the user interface; and restricting propagation of user interface events among the user interface objects, during presentation of the user interface objects in the user interface of the computer, based on node ownership information linking the user interface objects to the separate software applications. Other embodiments of this aspect include corresponding systems, apparatus, and computer program products.