A method of determining a layout of a user interface (UI) that composites elements from a plurality of applications, each of the plurality of applications having a respective UI client engine, the method comprising: receiving, from one of a plurality of UI client engines, a request to change layout parameters; negotiating, between the plurality of UI client engines, changes to the layout parameters to determine negotiated layout parameters; and transmitting the negotiated layout parameters to a UI rendering engine, as a component tree structure, for rendering to a display. A UI system, electronic device and computer program product are also provided.