在IE7+、Firefox中，由于支持了<option>的title属性，我们可以想办法给option标记设置title属性（内容可以与显示的值相同或者不同）。如果是已经做好的页面，不想再做太多改动，可以用下面的脚本，自动遍历页面上的所有<select>，给所有的option加上与text相同的title。
function SetOptionTitle()
{
var selects = document.getElementsByTagName("select"); if (selects.length > 0)
{
for (var i = 0; i < selects.length; i++) { var options = selects[i].options; if (selects[i].options.length > 0) { for (var j = 0; j < options.length; j++) { if (options[j].title == "") options[j].title = options[j].text;
} } } }
}我目前想到的办法是：当鼠标悬停到<select>时，创建一个这个下拉列表的副本，同时把焦点移到这个副本上，把副本的样式设成绝对定位，而且盖在原来的下拉列表上，宽度根据option的显示内容自动拉伸，当这个副本失去焦点，或者用户对它进行了选择操作后，获取副本的selectedIndex，赋给原来的select对象。具体代码如下：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<script type="text/javascript">
function FixWidth(selectObj)
{
var newSelectObj = document.createElement("select");
newSelectObj = selectObj.cloneNode(true);
newSelectObj.selectedIndex = selectObj.selectedIndex;
newSelectObj.onmouseover = null;
var e = selectObj;
var absTop = e.offsetTop;
var absLeft = e.offsetLeft;
while(e = e.offsetParent)
{
absTop += e.offsetTop;
absLeft += e.offsetLeft;
}
with (newSelectObj.style)
{
position = "absolute";
top = absTop + "px";
left = absLeft + "px";
width = "auto";
}
var rollback = function(){ RollbackWidth(selectObj, newSelectObj); };
if(window.addEventListener)
{
newSelectObj.addEventListener("blur", rollback, false);
newSelectObj.addEventListener("change", rollback, false);
}
else
{
newSelectObj.attachEvent("onblur", rollback);
newSelectObj.attachEvent("onchange", rollback);
}
selectObj.style.visibility = "hidden";
document.body.appendChild(newSelectObj);
newSelectObj.focus();
}
function RollbackWidth(selectObj, newSelectObj)
{
selectObj.selectedIndex = newSelectObj.selectedIndex;
selectObj.style.visibility = "visible";
document.body.removeChild(newSelectObj);
}
</script>
</head>
<body>
<form method="post">
<div style="100px; height:100px; margin:100px; padding:10px; background:gray;">
<select name="Select1" style="80px;" onmouseover="FixWidth(this)">
<option id="A" title="this is A">AAAAAAAAAAAAAAA</option>
<option id="B" title="this is B">BBBBBBBBBBBBBBB</option>
<option id="C" title="this is C">CCCCCCCCCCCCCCC</option>
</select>
</div>
</form>
</body>
</html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title></title>
<script type="text/javascript">
function FixWidth(selectObj)
{
var newSelectObj = document.createElement("select");
newSelectObj = selectObj.cloneNode(true);
newSelectObj.selectedIndex = selectObj.selectedIndex;
newSelectObj.onmouseover = null;
var e = selectObj;
var absTop = e.offsetTop;
var absLeft = e.offsetLeft;
while(e = e.offsetParent)
{
absTop += e.offsetTop;
absLeft += e.offsetLeft;
}
with (newSelectObj.style)
{
position = "absolute";
top = absTop + "px";
left = absLeft + "px";
width = "auto";
}
var rollback = function(){ RollbackWidth(selectObj, newSelectObj); };
if(window.addEventListener)
{
newSelectObj.addEventListener("blur", rollback, false);
newSelectObj.addEventListener("change", rollback, false);
}
else
{
newSelectObj.attachEvent("onblur", rollback);
newSelectObj.attachEvent("onchange", rollback);
}
selectObj.style.visibility = "hidden";
document.body.appendChild(newSelectObj);
newSelectObj.focus();
}
function RollbackWidth(selectObj, newSelectObj)
{
selectObj.selectedIndex = newSelectObj.selectedIndex;
selectObj.style.visibility = "visible";
document.body.removeChild(newSelectObj);
}
</script>
</head>
<body>
<form method="post">
<div style="100px; height:100px; margin:100px; padding:10px; background:gray;">
<select name="Select1" style="80px;" onmouseover="FixWidth(this)">
<option id="A" title="this is A">AAAAAAAAAAAAAAA</option>
<option id="B" title="this is B">BBBBBBBBBBBBBBB</option>
<option id="C" title="this is C">CCCCCCCCCCCCCCC</option>
</select>
</div>
</form>
</body>
</html>
有了这个demo，我们就可以把其中的js提取出来，应用到项目中需要的地方了。
目前这种方式是给select注册了onmouseover，对于鼠标操作是没什么问题，如果用户是按tab键移动焦点就无效了。估计实际应用中这样的情形不会很多，所以暂不考虑，有兴趣的朋友可以再完善一下，或者看看有没有更好的解决方法。
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多谢5楼兄弟的提醒，修改了demo，通过attachEvent/addEventListener添加事件后就不会覆盖原有的事件了。