iD/js/id/behavior/drag_node.js

iD.behavior.DragNode = function(mode) {
    var history = mode.history,
        size = mode.map.size(),
        nudgeInterval,
        projection = mode.map.projection;

    function edge(point) {
        var pad = [30, 100, 30, 100];
        if (point[0] > size[0] - pad[0]) return [-10, 0];
        else if (point[0] < pad[2]) return [10, 0];
        else if (point[1] > size[1] - pad[1]) return [0, -10];
        else if (point[1] < pad[3]) return [0, 10];
        return null;
    }

    function startNudge(nudge) {
        if (nudgeInterval) window.clearInterval(nudgeInterval);
        nudgeInterval = window.setInterval(function() {
            mode.map.pan(nudge).redraw();
        }, 50);
    }

    function stopNudge(nudge) {
        if (nudgeInterval) window.clearInterval(nudgeInterval);
        nudgeInterval = null;
    }

    return iD.behavior.drag()
        .delegate(".node")
        .origin(function(entity) {
            return projection(entity.loc);
        })
        .on('start', function() {
            history.perform(
                iD.actions.Noop());
        })
        .on('move', function(entity) {
            d3.event.sourceEvent.stopPropagation();

            var nudge = edge(d3.event.point);
            if (nudge) startNudge(nudge);
            else stopNudge();

            history.replace(
                iD.actions.MoveNode(entity.id, projection.invert(d3.event.point)),
                'moved a node');
        })
        .on('end', function() {
            stopNudge();
            history.replace(
                iD.actions.Noop(),
                'moved a node');
        });
};