HTML fieldset allows children to expand indefinitely

Question

I want to put a scrollable box inside of a fieldset, but I’ve run into a quirk and I can’t figure out my way around it. When you put your scrollable div inside of a fieldset, the fieldset expands instead of making the scrollable element scroll.

Here’s a test case. The following expands indefinitely (boo):

<div style="width: 300px; overflow: hidden;">
   <fieldset>
      <div style="overflow: scroll; white-space: nowrap;">
        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed lorem arcu, sodales non gravida eget, vehicula vitae nulla. Quisque turpis justo, consectetur ut egestas at, pulvinar nec diam. Donec porttitor lobortis elit quis scelerisque. Proin at mollis nibh. Nulla nisi dolor, rutrum nec rhoncus congue, cursus at urna. Curabitur adipiscing euismod nisl nec consequat. Aliquam justo justo, bibendum id molestie eget, dignissim sit amet sapien. Phasellus non erat nulla, quis auctor eros. Proin pellentesque turpis eu ipsum venenatis egestas non eget lacus. Vestibulum ante diam, posuere ut fringilla nec, pretium ac metus. Integer laoreet fringilla ipsum, vel interdum urna pellentesque a. Donec lobortis tincidunt nisi, ac tristique massa pretium ac. Ut vel magna erat, et hendrerit sem. Curabitur vulputate, tellus quis pellentesque pretium, felis odio aliquam sapien, sit amet hendrerit arcu orci ut nulla. Vestibulum suscipit rhoncus arcu, ut aliquam eros sagittis a. Suspendisse eros elit, bibendum venenatis pulvinar at, scelerisque vel quam. 
      </div>
   </fieldset>
</div>

But if you use a div, it works as expected (hurray!):

<div style="width: 300px; overflow: hidden;">
   <div>
      <div style="overflow: scroll; white-space: nowrap;">
        Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed lorem arcu, sodales non gravida eget, vehicula vitae nulla. Quisque turpis justo, consectetur ut egestas at, pulvinar nec diam. Donec porttitor lobortis elit quis scelerisque. Proin at mollis nibh. Nulla nisi dolor, rutrum nec rhoncus congue, cursus at urna. Curabitur adipiscing euismod nisl nec consequat. Aliquam justo justo, bibendum id molestie eget, dignissim sit amet sapien. Phasellus non erat nulla, quis auctor eros. Proin pellentesque turpis eu ipsum venenatis egestas non eget lacus. Vestibulum ante diam, posuere ut fringilla nec, pretium ac metus. Integer laoreet fringilla ipsum, vel interdum urna pellentesque a. Donec lobortis tincidunt nisi, ac tristique massa pretium ac. Ut vel magna erat, et hendrerit sem. Curabitur vulputate, tellus quis pellentesque pretium, felis odio aliquam sapien, sit amet hendrerit arcu orci ut nulla. Vestibulum suscipit rhoncus arcu, ut aliquam eros sagittis a. Suspendisse eros elit, bibendum venenatis pulvinar at, scelerisque vel quam. 
      </div>
   </div>
</div>

How can I get the fieldset to behave like the div?

Answer 1

WebKit and Firefox constrain fieldsets to have an "implicit" width based on the computed width of their contents. Here's how to override it in each.

• WebKit makes it relatively easy. This behaviour is defined in the default stylesheet, so you can override it by specifying min-width: 0 for the fieldset.

• Firefox is a tougher nut because fieldset width constraints are enforced deep in the Gecko layout code. Fortunately, there is a workaround: add a Gecko-only rule to set the display property for the fieldset to a value corresponding to one of several internal table elements.

Putting it all together:

fieldset { min-width: 0; }

/* FF hack; not needed for newer versions */
@-moz-document url-prefix() { /* Only target Gecko. (Breaks layout in IE.) */
    fieldset { display: table-cell; }
}

jsFiddle demo.

Update (25 Sept 2017)

The [Firefox bug][bug] that necessitated the display: table-cell hack has been fixed now; if you are targetting newer versions of Firefox, you may omit that and just use min-width: 0. (Thanks @Nikolay for the reminder!)

For more details on why that used to be needed, my answer to a related question has a fuller explanation, including the gory browser internals.