1. A method for integrating separate software applications in an application execution environment, the method comprising: 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; wherein the providing comprises 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; wherein the restricting comprises, when performing an event dispatch: checking the node ownership information for each node in a path from a target node to a root node in the hierarchical structure, the target node corresponding to a user interface object currently having an input focus; and limiting the event dispatch at an inter-node point in the path where a mutual trust relationship is lacking, with respect to the event dispatch, between an owner of a current node and an owner of the target node.