chrome 中如何处理setCapture

function draggable(element) {
    var dragging = null;

    addListener(element, "mousedown", function(e) {
        var e = window.event || e;
        dragging = {
            mouseX: e.clientX,
            mouseY: e.clientY,
            startX: parseInt(element.style.left),
            startY: parseInt(element.style.top)
        };
        if (element.setCapture) element.setCapture();
    });

    addListener(element, "losecapture", function() {
        dragging = null;
    });

    addListener(document, "mouseup", function() {
        dragging = null;
    }, true);

    var dragTarget = element.setCapture ? element : document;

    addListener(dragTarget, "mousemove", function(e) {
        if (!dragging) return;

        var e = window.event || e;
        var top = dragging.startY + (e.clientY - dragging.mouseY);
        var left = dragging.startX + (e.clientX - dragging.mouseX);

        element.style.top = (Math.max(0, top)) + "px";
        element.style.left = (Math.max(0, left)) + "px";
    }, true);
};

draggable(document.getElementById("drag"));