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問題：fixed元素被另一個fixed元素包含的時候在chrome下fixed子項目的定位會受到父元素的影響。
demo（http://jsbin.com/qumah/1）：
<!DOCTYPE html><html><head> <meta charset="utf-8"> <title>JS Bin</title> <style> .fixed { position: fixed; left:0; top:20px; width:200px; height:200px; background:#fee; } .fixed-child { width:150px; height:150px; background: #fcc; top:40px; right: 20px; left: auto; z-index: 100; } .absolute { position: absolute; right: 20px; width:100px; height:100px; background:#f11; } </style></head><body> <div class="fixed">fixed position<div class="fixed fixed-child">fixed child position</div></div> <div class="absolute">absolute position</div> </body></html>
IE（IE8-IE11效果一致，IE7沒有測試，IE6不支援fixed元素）下效果：
chrome下效果：
firefox下效果：
Safari下效果：
解釋：層疊關係是受層疊上下文影響的。文檔中的層疊上下文由滿足以下任意一個條件的元素形成：
opacity屬性值小於1. (參考 the specification for opacity),position: fixed 總是建立一個新的層疊上下文, 即使z-index的值是"auto" (See this post)參考來源：The stacking context - MDN
每個層疊上下文都有如下的層疊層級組成（顯示順序從後到前）：
參考來源：RM8015: IE6 IE7 IE8(Q) 中定位元素 'z-index' 為預設值在某些情況下會產生新的層疊上下文
結論：在chrome22+的瀏覽器中，position為fixed總是會建立新的重疊上下文，所以子fixed元素在此時會以父fixed元素為層疊上下文，子項目的層疊關係會受到父元素的影響。而在非chrome瀏覽器下子fixed元素並不會建立新的層疊上下文，fixed元素的層疊上下文為最近的層疊上下文。
因此，我們應該盡量避免出現fixed元素相互嵌套的問題。如果必須有嵌套的情況，建議修改fixed父元素的z-index值來修正在chrome下fixed子項目的層疊問題。
題外話：
對於opacity小於1的元素也會產生層疊內容相關的問題，可能很多人都不知道。但是規範裡面是明文規定的：
Since an element with opacity less than 1 is composited from a single offscreen image, content outside of it cannot be layered in z-order between pieces of content inside of it. For the same reason, implementations must create a new stacking context for any element with opacity less than 1. If an element with opacity less than 1 is not positioned, implementations must paint the layer it creates, within its parent stacking context, at the same stacking order that would be used if it were a positioned element with ‘
z-index: 0’ and ‘opacity: 1’. If an element with opacity less than 1 is positioned, the ‘z-index’ property applies as described in [CSS21], except that ‘auto’ is treated as ‘0’ since a new stacking context is always created. See section 9.9 andAppendix E of [CSS21] for more information on stacking contexts. The rules in this paragraph do not apply to SVG elements, since SVG has its own rendering model ([SVG11], Chapter 3). ---摘自《CSS Color Module Level 3》
簡單來說：
參考文章：
https://developer.mozilla.org/zh-CN/docs/Web/CSS/Understanding_z-index/The_stacking_context
http://www.w3.org/TR/CSS21/visuren.html#z-index
https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index
RM8015: IE6 IE7 IE8(Q) 中定位元素 'z-index' 為預設值在某些情況下會產生新的層疊上下文
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