Disclosed are examples of systems, apparatus, methods and computer program products for providing a user interface customizable to define state transitions of panes in a bounding window. For example, a bounding window including one or more resizable panes is displayed in a user interface on a display device. Breakpoints situated at respective spatial positions in the bounding window are graphically represented in the user interface. Each breakpoint defines a transition of a pane from one state to another when the pane is resized to traverse the breakpoint. The spatial position of one or more of the breakpoints can be updated in accordance with a request indicating user customization of breakpoints.