Craft 3 - custom callback after Garnish onDragStop
Garnish’s Base class introduces its own concept of events, that is separate from DOM events.

Here’s how you’d set up the listener:

// get the Craft.MatrixInput object
var matrix = $('.matrix-field').data('matrix');

// attach a handler to its block sorter's sortChange event
matrix.blockSort.on('sortChange', function() {
    // event handler...
});