onMouseMiddleClick property Null safety
Dispatched when a user presses and releases the middle button of the user's pointing device over the same InteractiveObject.
For a middleClick event to occur, it must always follow this series of events in the order of occurrence: middleMouseDown event, then middleMouseUp. The target object must be identical for both of these events; otherwise the middleClick event does not occur. Any number of other mouse events can occur at any time between the middleMouseDown or middleMouseUp events; the middleClick event still occurs.
EventStream<MouseEvent> get onMouseMiddleClick => InteractiveObject.mouseMiddleClickEvent.forTarget(this);