html select下拉菜单长度过长不出滚动条问题解决办法

<select class="form-control"  data-edit-select="1"  onmousedown="selectmousedown(this)" 
onblur="selectblur(this)" onchange="selectchange(this)" 
style="position:absolute;z-index:120"> 
</select>

function selectmousedown(ev) {
    if (ev.options.length > 5) {
        ev.size = 6;
        $(ev).css({
            "height": "auto",
            "position": "absolute"
        })
    } else {
        $(ev).css("height", "28px")
    }
}

function selectblur(ev) {
    ev.size = 0;
    $(ev).css({
        "height": "28px",
        "position": "static"
    })
}

function selectchange(ev) {

    ev.size = 0;
    $(ev).css({
        "height": "28px",
        "position": "static"
    })
}

　　

结果如图：