This tool can start when the middle mouse button is pressed, unless AutoPan is false (for manual panning), in which case the left mouse button is used.
However, for manual panning on WebForms, the left mouse button is expected only when Modal is true. When used modelessly, the middle button must be used, since the whole operation will consist of a background-mouse-down and a mouse-up, and that gesture will not be confused with a background rubber-band selection.