The Boundary link tool renders a rectangular border to show the bounding box of the link. It accepts one additional argument, which can be passed as an object to the link tool constructor:

padding number This option determines whether the boundary area should be visually inflated and if so, by how much. Default is 10.

Example:

var boundaryTool = new joint.linkTools.Boundary({
    focusOpacity: 0.5,
    padding: 20
});