Question

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

Answer 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