OptionalgetElement whose getBoundingClientRect() defines the local coordinate
space. Defaults to the handle target's owning <svg> if it has one,
otherwise the target itself.
OptionalonCalled on pointerup / pointercancel, after listeners are removed.
Called on every pointermove with coords local to the rect element (see getRect).
OptionalonCalled on pointerdown with the same local coords passed to onMove.
Options for useHandleDrag.