This sample demonstrates the LinkShiftingTool, which is an extra tool that can be installed in the ToolManager to allow users to shift the end point of the link to be anywhere along the sides of the port with which it remains connected. This extension tool is defined in its own file, as LinkShiftingTool.js.
This only looks good for ports that occupy the whole of a rectangular node.
If you want to restrict the user's permitted sides, you can adapt the
LinkShiftingTool.doReshape method to do what you want.