parent & child with position fixed, parent overflow:hidden bug

问题

I don\'t know if there is an issue, but I was wondering why the overflow:hidden does not function on fixed parent/children element.

Here\'s an example:

CSS and HTML:

.parent{
  position:fixed;
  overflow:hidden;
  width:300px;
  height:300px;
  background:#555;
}
.children{
  position:fixed;
  top:200px;
  left:200px;
  width:150px;
  height:150px;
  background:#333;
}

<div class=\"parent\">
  <div class=\"children\">
  </div>
</div>

Live demo: jsFiddle

回答1:

Because a fixed position element is fixed with respect to the viewport, not another element. Therefore since the viewport isn't cutting it off, the overflow becomes irrelevant.

Whereas the position and dimensions of an element with position:absolute are relative to its containing block, the position and dimensions of an element with position:fixed are always relative to the initial containing block. This is normally the viewport: the browser window or the paper’s page box.

ref: http://www.w3.org/wiki/CSS_absolute_and_fixed_positioning#Fixed_positioning

回答2:

You could consider using CSS clip: rect(top, right, bottom, left); to clip a fixed positioned element to a parent. See demo at http://jsfiddle.net/lmeurs/jf3t0fmf/.

Beware, use with care!

Though the clip style is widely supported, main disadvantages are that:

The parent's position cannot be static or relative (one can use an absolutely positioned parent inside a relatively positioned container);
The rect coordinates do not support percentages, though the auto value equals 100%, ie. clip: rect(auto, auto, auto, auto);;
Possibillities with child elements are limited in at least IE11 & Chrome34, ie. we cannot set the position of child elements to relative or absolute or use CSS3 transform like scale.

See http://tympanus.net/codrops/2013/01/16/understanding-the-css-clip-property/ for more info.

EDIT: Chrome seems to handle positioning of and CSS3 transforms on child elements a lot better when applying backface-visibility, so just to be sure we added:

-webkit-backface-visibility: hidden;
-moz-backface-visibility: hidden;
backface-visibility: hidden;

to the main child element.

Also note that it's not fully supported by older / mobile browsers or it might take some extra effort. See our implementation for the menu at bellafuchsia.com.

IE8 shows the menu well, but menu links are not clickable;
IE9 does not show the menu under the fold;
iOS Safari <5 does not show the menu well;
iOS Safari 5+ repaints the clipped content on scroll after scrolling;
FF (at least 13+), IE10+, Chrome and Chrome for Android seem to play nice.

EDIT 2014-11-02: Demo URL has been updated.

回答3:

2016 update:

You can create a new stacking context, as seen on Coderwall:

<div style="transform: translate3d(0,0,0);overflow:hidden">
   <img style="position:fixed; ..." />
</div>

Which refers to http://dev.w3.org/csswg/css-transforms/#transform-rendering

For elements whose layout is governed by the CSS box model, any value other than none for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants.

回答4:

As an alternative to using clip you could also use {border-radius: 0.0001px} on a parent element. It works not only with absolute/fixed positioned elements.

回答5:

If you want to hide overflow on fixed-position elements, the simplest approach I've found is to place the element inside a container element, and apply position:fixed and overflow:hidden to that element instead of the contained element (you must remove position:fixed from the contained element for this to work). The content of the fixed container should then be clipped as expected.

In my case I was having trouble with using object-fit:cover on a fixed-position element (it was spilling outside the bounds of the page body, regardless of overflow:hidden). Placing it inside a fixed container with overflow:hidden on the container fixed the issue.

回答6:

I had a similar, quite complex problem with a fluid layout, where the right column had a fixed width and the left one had a flexible width. My fixed container should have the same width as the flexible column. Here is my solution:

HTML

<div id="wrapper">
    <div id="col1">
    <div id="fixed-outer">
        <div id="fixed-inner">inner</div>
    </div>
    COL1<br />Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.
    </div>
    <div id="col2">COL2</div>
</div>

CSS

    #wrapper {
    padding-left: 20px;
}

#col1 {
    background-color: grey;
    float: left;
    margin-right: -200px; /* #col2 width */
    width: 100%;
}

#col2 {
    background-color: #ddd;
    float: left;
    height: 400px;
    width: 200px;
}

#fixed-outer {
    background: yellow;
    border-right: 2px solid red;
    height: 30px;
    margin-left: -420px; /* 2x #col2 width + #wrapper padding-left */
    overflow: hidden;
    padding-right: 200px; /* #col2 width */
    position: fixed;
    width: 100%;
}

#fixed-inner {
    background: orange;
    border-left: 2px solid blue;
    border-top: 2px solid blue;
    height: 30px;
    margin-left: 420px; /* 2x #col2 width + #wrapper padding-left */
    position: absolute;
    width: 100%;
}

Live Demo: http://jsfiddle.net/hWCub/