jQuery - Animate element that has children absolutely positioned outside it - blinking

问题

Forgive me if this has been addressed before, couldn't find anything.

I am animating a content bar that that has children absolutely positioned outside it (via negative margins). The idea is that the children will animate with the bar as it expands.

What happens is as soon as the animation starts the children disappear, and then reappear when the animation is complete. It's as if the animation need to complete before the browser knows where to put the children.

I have uploaded a really simple example here, all scripts included on page: http://www.ismailshallis.com/jdemo/

What is actually happening? What are my options to work around this?

Many thanks in advance,

Belinda

回答1:

When jquery is animating either the height or width of an element, it will automatically set overflow: hidden on the element while the animation is happening. Since your child element is positioned outside, it's technically part of the overflow. The comment in the jquery source near the code that does this says "//Make sure that nothing sneaks out." If you include the uncompressed jquery source and comment out line 4032 of jquery-1.3.2.js (inside the animate function):

//this.style.overflow = "hidden";

You will see that the animation works the way you intended. I'm not sure of a workaround other than modifying the jquery source by commenting out that line above.

回答2:

As of jQuery 1.4.3 there is another solution that does not require modifying jQuery. If you set the 'overflow' style of the element that you are animating as an inline style before starting the animation then jQuery will not set the 'overflow' style to hidden. For example:

<script type="text/javascript">
    $(document).ready(function() {
        $("#box a").click(function() {
            $("#box")

            // Prevents absolutely positioned elements from getting clipped.
            .css('overflow', 'visible')

            .animate({
                height: "410px"
            })

            // Reset the 'overflow' style.  You could also put this in the 
            // animate() callback.
            .css('overflow', '');
        });
    });
</script>

回答3:

Well - it seems that it's a function of the browser or jQuery, not necessarily of the way you've constructed your HTML or Javascript. I say this because it seems only the pixel area inside the bounding box of the DOM element seems to be rendered (try moving the text so that half of the text is outside, and half inside... you see a "cut off" piece of text as it animates.)

So here's the work around: It uses a wrapper DIV around "#box" and "#outside" such that both are inside the wrapper's bounding box.

CSS:

    #boxWrapper {
        position: fixed;
        top: 50%;
        left:  50%;
        width: 200px;
        height: 200px;
        margin-left: -100px;
        margin-top: -120px; /* extra 20px for the #outside */
        background:#ccc;
    }

    #box {
        background: #000;
        height:100%;
        width:100%;
        margin-top:20px; /* give 20px for the #outside */
    }

    #outside {
        background:darkblue;
        position: absolute;
        top: 0px;
        right: 0; }

And the HTML:

<div id="boxWrapper">
    <div id="box">
        <a href="#">CLICK ME</a>
        <div id="outside">
            I'm positioned OUTSIDE the box
        </div>
    </div>
</div>

And the Javascript:

<script type="text/javascript">
    $(document).ready(function() {

            $("#box a").click(function(){
                $("#boxWrapper").animate({ 
                        height: "410px",
                        opacity: 0.9,
                        top: "25%"
                      }, 1000 );
                return false;
            });
        }); 

</script>   

回答4:

Alternatively, you can indicate your preference that the element remain visible by using the !important specification.

#box {
  overflow: visible !important;
}

来源：https://stackoverflow.com/questions/1085206/jquery-animate-element-that-has-children-absolutely-positioned-outside-it-bl