今天會討論到這個課題，是因為要實做一個Popup dialog，所以我們希望的結果如下圖。
可是在IE7 卻發生了這樣的情況。
Popup不論怎麼設定z-index都無法浮在最上層，我們看一下html架構發生什麼事情。
從這邊可以發現，在IE 7裡面因為overlay 的上一個層級 page，有設定relative，導致overlay有繼承行為，因此讓overlay的z-index無法作用。
這樣子思考起來除了IE 7以外的瀏覽器(chrome、safari、Opera、IE 8...)的z-index繼承邏輯似乎有錯誤，沒有看錯吧！？z-index 在IE 7才是標準？
為了幫其他瀏覽器證明一下他們是有繼承概念的，因此幫幾個div加上z-index。
修改的部份，如紅字所示，在mask、page都加上了z-index: 0;，輸出結果如下圖。
經過測試後發現，popup已經沉下去mask底下了，表示其他瀏覽器還是會有繼承z-index的動作，因此有底下幾點推論：
可是在IE7 卻發生了這樣的情況。
Popup不論怎麼設定z-index都無法浮在最上層，我們看一下html架構發生什麼事情。
<!DOCTYPE html>
<html>
<head>
<title>Demo mask overlay</title>
<link rel="stylesheet" type="text/css" href="http://yui.yahooapis.com/3.2.0/build/cssreset/reset-min.css">
<style>
#mask {
width: 100%;
height: 100%;
background: #000;
position: absolute;
top: 0;
left: 0;
/* other browser*/
opacity: 0.5;
/* IE 5-7 */
filter: alpha(opacity=50);
/* IE 8 */ -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
}
#page {
position: relative;
border: 1px solid #ccc;
background: #ccc;
top: 20px;
left: 100px;
width: 80%;
height: 50px;
}
#page .overlay {
position: absolute;
top: 50%;
left: 50%;
width: 200px;
height: 200px;
border: 1px solid #000;
background: #fff;
-webkit-border-radius: 5px;
-moz-border-radius: 5px;
border-radius: 5px;
color: #999;
font-size: 131%;
z-index: 1;
}
</style>
</head>
<body>
<div id="page">
<h1> I am div </h1>
<div class="overlay">
<p>This is popup. </p>
</div>
</div>
<div id="mask">
</div>
</body>
</html>
從這邊可以發現，在IE 7裡面因為overlay 的上一個層級 page，有設定relative，導致overlay有繼承行為，因此讓overlay的z-index無法作用。
這樣子思考起來除了IE 7以外的瀏覽器(chrome、safari、Opera、IE 8...)的z-index繼承邏輯似乎有錯誤，沒有看錯吧！？z-index 在IE 7才是標準？
為了幫其他瀏覽器證明一下他們是有繼承概念的，因此幫幾個div加上z-index。
<!DOCTYPE html>
<html>
<head>
<title>Demo mask overlay</title>
<link rel="stylesheet" type="text/css" href="http://yui.yahooapis.com/3.2.0/build/cssreset/reset-min.css">
<style>
#mask {
width: 100%;
height: 100%;
background: #000;
position: absolute;
top: 0;
left: 0;
/* other browser*/
opacity: 0.5;
/* IE 5-7 */
filter: alpha(opacity=50);
/* IE 8 */ -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity=50)";
z-index: 0;
}
#page {
position: relative;
border: 1px solid #ccc;
background: #ccc;
top: 20px;
left: 100px;
width: 80%;
height: 50px;
z-index: 0;
}
#page .overlay {
position: absolute;
top: 50%;
left: 50%;
width: 200px;
height: 200px;
border: 1px solid #000;
background: #fff;
-webkit-border-radius: 5px;
-moz-border-radius: 5px;
border-radius: 5px;
color: #999;
font-size: 131%;
z-index: 1;
}
</style>
</head>
<body>
<div id="page">
<h1> I am div </h1>
<div class="overlay">
<p>This is popup. </p>
</div>
</div>
<div id="mask">
</div>
</body>
</html>
修改的部份，如紅字所示，在mask、page都加上了z-index: 0;，輸出結果如下圖。
經過測試後發現，popup已經沉下去mask底下了，表示其他瀏覽器還是會有繼承z-index的動作，因此有底下幾點推論：
- IE 7會將父層z-index預設為0。
- 其他瀏覽器不會預設z-index。
- 並不是z-index設定值越大就一定會在最上層，同時考慮繼承問題。
- 使用浮動在最上層的獨立div，盡量靠近body層級。
撰寫客戶網站時剛好碰到這問題.
回覆刪除看來似乎如您所說的會將父級設為0
顧我將其餘的區域設為負數便終於解決了
謝謝您分享
@SHAWN, 蠻多時候使用div定位，多少都會遇到z-index，尤其是層層包覆的時候特別常見，感謝您的回應。
回覆刪除另外我不確定您的問題，其實可以測試看看把某個層級div的z-index: 1;只要比其他高，其實就可以浮在最上層，試試看。
找到了很細節的魔鬼，真不簡單! =b
回覆刪除其實照 W3C 的規範，IE7 跟 FF 都沒有錯。
IE7 對 z-index 的預設值是 0、而 FF 則是 auto。http://josephj.com/lab/ie7-zindex/demo.html
真的在負責處理 z-index 的顯示叫 Stacking Context。W3C 的文件中有一句話「The root element forms the root stacking context. Other stacking contexts are generated by any positioned element (including relatively positioned elements) having a computed value of 'z-index' other than 'auto'.」如果你設定了 position:fixed|absolute|relative、但卻沒有設定 z-index 為 auto 以外的數值的話，它對應的 Stacking Context 還是根節點。所以此時就會直接比 #mask 與 #overlay 的值，.overlay 不管 #page 是什麼碗糕、直接參考根節點，因為 z-index:1，所以 .overlay 直接勝出。http://www.w3.org/TR/CSS2/visuren.html#z-index
繼承的論點很怪，z-index 是不會繼承的，他只會依照此元素的 Stacking Context 與其他在此 Stacking Context 的元素做比對、做出正確的排序 。
我在開發時沒有碰過這樣的問題，我會先把這樣要浮在上面的模組往根節點去丟（document.body.appendChild 或一開始就放在對的地方），也一律設定節點的 position 與 z-index 屬性，讓結構清楚減少困擾。http://www.tjkdesign.com/articles/z-index/teach_yourself_how_elements_stack.asp
＠josephj
回覆刪除如果使用overlay的話，也的確需要使用上javascript(因此無法考慮js disable狀態)，因此採用document.body.appendChild，將overlay div 搬移的確是一個不錯的方法。
關於z-index的繼承問題，為了釐清真相還是採用瀏覽器實測，果然處處都是火坑啊，這一切的一切都要感謝我後座的夥伴，他把這個問題發掘，讓我對於z-index認知更深入瞭解，感謝各位夥伴！=b
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