code: function onMouseDown (ev) {
        var canvasOffset = display._getCanvasOffset();
        _triggerCallbacks({
           'type': exports.MOUSE_DOWN,
           'pos': [ev.clientX - canvasOffset[0], ev.clientY - canvasOffset[1]],
           'button': ev.button,
           'shiftKey': ev.shiftKey,
           'ctrlKey': ev.ctrlKey,
           'metaKey': ev.metaKey
        });
     }
docs: anonymous functions as event handlers = memory leak, see MDC:elementAddEventListener