Hiding div behind its parent?

问题

<div class="content-wrapper">
    <div class="popup">
        <div class="close">
        </div>
    </div>
</div>

.content-wrapper is relatively positioned and contains all the page content (not just the popup).

.popup is absolutely positioned.

.close is also absolutely positioned.

I have some javascript to move close when the cursor enters popup (so I have a nice close bar appear out the side). The best way I have found to do this is just to move using jQuery animate. Hiding/showing creates a stuttering affect even .stop() wasn't able to solve. My problem is in trying to hide .close behind .popup. No matter what z-index I set for the two divs .close will not sit behind .popup.

Is it possible to have an absolutely positioned div inside another absolutely positioned div sit behind its parent, and if so how?

回答1:

Yep, use z-index: http://jsfiddle.net/tGd4Q/

HTML:

<div class="content-wrapper">
    <div class="popup">
        <div class="close">
        </div>
    </div>
</div>​

CSS:

.popup, .close { position: absolute; height: 200px; width: 200px; }
.popup { background: #f00; }
.close { background: #ff0; top: 25px; left: 25px; z-index: -1; }​

This won't work with IE7 standards though. I suggest using jQuery(or other framework of your choosing) to hide the div:

$('.popup .close').hide();

回答2:

Stacking indices are most of the time relative to siblings, so you cannot put a child behind it's parent using z-index.

Here is some more information about that.

This is the stacking order:

1. The borders and background of the current stacking context
2. Positioned descendants with negative z-index
3. Nonpositioned block-level descendants with no z-index property defined -- paragraphs, tables, lists, and so on
4. Floating descendants and their contents
5. Nonpositioned inline content
6. Positioned descendants with no z-index, z-index: auto, or z-index: 0
7. Positioned descendants with z-index greater than 0

Nick McCormack uses z-index: -1 in his answer. This is indeed one exception to what your feelings give in. Beware that z-index: -1 moves an element behind many of your elements to the background.

Browser differences

Beside that, Internet Explorer does not support negative stacking indices and is very strict with element (child/parent) positions. Every element level has it's own stacking context, so have to 'communicate' via the parent element. See this explanation.

According to Smashing Magazine, the select element, which is a windowed control, has naturally a higher stacking index.

According to the Shadowbox troubleElement option, I presume that object, embed and canvas have the same issues.

---

If you want to hide .close, why don't you really hide it instead of moving it behind .popup?

$('.close').hide();

回答3:

No, you will not be able to put it behind its parent. However you could change its display mode to none, so it isn't seen at all. Then when you need to see the div, change it to show.

Simple jQuery:

$('.close').hide();
$('.close').show();

There are other ways as well, such as adding an attribute of style with display:none or display: inline-block as a setting.

Update: According to comments in other answers, there IS a way to do it with z-index. Still thinking the hide/show is the way to go though. Very clear what you are doing on your UI.

来源：https://stackoverflow.com/questions/10030386/hiding-div-behind-its-parent