The name MousePressed has been in use before right clicks were supported at all, which is why the name doesn't make it clear that it only refers to left mouse clicks.
Support for right mouse clicks is currently extremely limited. Widgets know when the right mouse is pressed with the internal rightMousePressed and rightMouseReleased functions, but ListView is currently the only widget with a RightClicked callback.
What are you trying to do exactly? Would you need a callback on the actual click, or already when the right mouse button goes down?