Why does `overflow:hidden` prevent `position:sticky` from working?

Question

In the following snippet, there is a sticky div positioned inside a container. It sticks to the top of the scrolling panel while staying inside its container all the time. This is the same behavior as that of the UITableView headers on iOS, where the headers stay visible until the next header is at the top.

In the second snippet, everything is the same except that the container has an overflow:hidden CSS rule. This seems to prevent the position:sticky behavior from working correctly.

.parent {    position: relative;    background: #ccc;    width: 500px;    height: 150px;    overflow: auto;    margin-bottom: 20px;  }    .hidden-overflow {    overflow: hidden;  }    .sticky {    position: sticky;    background: #333;    text-align: center;    color: #fff;    top: 10px;  }

<div class="parent">    <div>      <div class="sticky">        Hi, I am a sticky inside the container which contains the first paragraph.      </div>      <p>        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus volutpat sed metus et porttitor. Integer bibendum lacus eget massa ultricies fermentum. Donec cursus magna eu congue posuere. Sed eget ligula quam. Sed laoreet enim sapien, eget volutpat nisl pellentesque vel. Nulla id dolor sed dolor sodales tristique. Curabitur feugiat massa sed massa bibendum semper et ac orci. In imperdiet nibh quis iaculis viverra. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Quisque vestibulum, nunc non volutpat tristique, nisl nisi volutpat nibh, quis pulvinar purus ex nec justo. Sed a cursus turpis. Quisque nulla odio, lacinia quis vestibulum sit amet, elementum laoreet nisi. Etiam aliquet ligula sagittis,      consectetur ipsum sit amet, sodales augue.      </p>    </div>    <p>      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus volutpat sed metus et porttitor. Integer bibendum lacus eget massa ultricies fermentum. Donec cursus magna eu congue posuere. Sed eget ligula quam. Sed laoreet enim sapien, eget volutpat      nisl pellentesque vel. Nulla id dolor sed dolor sodales tristique. Curabitur feugiat massa sed massa bibendum semper et ac orci. In imperdiet nibh quis iaculis viverra. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos      himenaeos. Quisque vestibulum, nunc non volutpat tristique, nisl nisi volutpat nibh, quis pulvinar purus ex nec justo. Sed a cursus turpis. Quisque nulla odio, lacinia quis vestibulum sit amet, elementum laoreet nisi. Etiam aliquet ligula sagittis,      consectetur ipsum sit amet, sodales augue.    </p>    <p>      Integer congue augue a quam tincidunt, vitae dictum sem iaculis. Proin feugiat nibh vitae leo facilisis, eget laoreet augue dictum. Nunc facilisis tempor feugiat. Aenean eget interdum diam. Maecenas non risus iaculis, scelerisque ipsum eu, facilisis urna.      Integer velit justo, vestibulum vel vulputate vel, bibendum eu lorem. Phasellus viverra nisl a mi pretium eleifend.    </p>  </div>  <div class="parent">    <div class="hidden-overflow">      <div class="sticky">        Hi, I am another sticky in the container which contains the first paragraph, but my container has overflow:hidden.      </div>      <p>        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus volutpat sed metus et porttitor. Integer bibendum lacus eget massa ultricies fermentum. Donec cursus magna eu congue posuere. Sed eget ligula quam. Sed laoreet enim sapien, eget volutpat nisl pellentesque vel. Nulla id dolor sed dolor sodales tristique. Curabitur feugiat massa sed massa bibendum semper et ac orci. In imperdiet nibh quis iaculis viverra. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Quisque vestibulum, nunc non volutpat tristique, nisl nisi volutpat nibh, quis pulvinar purus ex nec justo. Sed a cursus turpis. Quisque nulla odio, lacinia quis vestibulum sit amet, elementum laoreet nisi. Etiam aliquet ligula sagittis,      consectetur ipsum sit amet, sodales augue.      </p>    </div>    <p>      Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus volutpat sed metus et porttitor. Integer bibendum lacus eget massa ultricies fermentum. Donec cursus magna eu congue posuere. Sed eget ligula quam. Sed laoreet enim sapien, eget volutpat      nisl pellentesque vel. Nulla id dolor sed dolor sodales tristique. Curabitur feugiat massa sed massa bibendum semper et ac orci. In imperdiet nibh quis iaculis viverra. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos      himenaeos. Quisque vestibulum, nunc non volutpat tristique, nisl nisi volutpat nibh, quis pulvinar purus ex nec justo. Sed a cursus turpis. Quisque nulla odio, lacinia quis vestibulum sit amet, elementum laoreet nisi. Etiam aliquet ligula sagittis,      consectetur ipsum sit amet, sodales augue.    </p>    <p>      Integer congue augue a quam tincidunt, vitae dictum sem iaculis. Proin feugiat nibh vitae leo facilisis, eget laoreet augue dictum. Nunc facilisis tempor feugiat. Aenean eget interdum diam. Maecenas non risus iaculis, scelerisque ipsum eu, facilisis urna.      Integer velit justo, vestibulum vel vulputate vel, bibendum eu lorem. Phasellus viverra nisl a mi pretium eleifend.    </p>  </div>

_{(Snippet adapted from @Daniel's here)}

Why doesn't position:sticky work in a container with overflow:hidden?

Answer 1

overflow: hidden is not preventing position: sticky from working. But if you set overflow to hidden on any ancestor of your sticky element, then this ancestor element will be the scrolling container for your sticky element. If you switch the overflow value on your ancestor from hidden to scroll and scroll this ancestor (not the window), then you can see that sticky is still working.

See also https://github.com/wilddeer/stickyfill#pro-tips:

Any non-default value (not visible) for overflow, overflow-x, or overflow-y on any of the predecessor elements anchors the sticky to the overflow context of that predecessor. Simply speaking, scrolling the predecessor will cause the sticky to stick, scrolling the window will not. This is expected with overflow: auto and overflow: scroll, but often causes problems and confusion with overflow: hidden.

Or http://www.coreyford.name/files/position-sticky-presentation/:

The box's position depends on its containing block (established as for position:static) as well as its scrolling container, defined by the nearest ancestor in the same document with a computed value for 'overflow-x' or 'overflow-y' other than 'visible', or the viewport if no such ancestor exists.

Or the CSS Positioned Layout Module Level 3 W3C Working Draft:

A stickily positioned box is positioned similarly to a relatively positioned box, but the offset is computed with reference to the nearest ancestor with a scrolling box, or the viewport if no ancestor has a scrolling box.