html select下拉菜單長度過長不出滾動條問題解決辦法

<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"
    })
}

　　

結果如圖：