Foolproof Flexbox: Fixing Flowing Frustrations

I recently encountered a frustrating issue while using flexbox for a 2 column layout on Brush Ninja. One column could have any width, while the other had a fixed width for a skyscraper advert. However, I faced a problem where the flexible-width column would overflow and make the content too wide. This issue only arose after adding the skyscraper advert.

To be honest, this problem caused quite a panic for me. I didn’t realize it was happening until after I published the website to production.

Initially, I considered using CSS grid and minmax to prevent overflows. However, I wanted my layout to dynamically adjust and resize when removing the advertisement. Flexbox seemed like a better fit for my needs.

After some trial and error, I stumbled upon the fix for flexbox that was closely related to using minmax. Adding min-width:0 did the trick! Initially, I tried adding max-width, but that didn’t restrict the size.

It turns out that by default, HTML elements have min-width:0, unless you’re using flexbox or CSS grid - in which case it is set to min-width:auto. This auto value allows items to overflow their containers. By explicitly setting min-width:0 on the columns we want to contain, we can resolve this issue once and for all.

Understanding Why It Works

When using flexbox or CSS grid layouts, elements are given an automatic minimum width (min-width:auto). This means they can exceed their container’s boundaries if necessary. While this behavior can possibly be useful in certain scenarios (I have yet to think of any), it’s a problem when we want our content to stay within specific limits.

By adding min-width:0 to our desired columns within flexbox, we override the default behavior and ensure that they do not overflow. This simple adjustment enables our content to remain within the designated width, even if we dynamically remove or resize elements.

I’m thinking about changing my CSS framework, ElementalCSS, to force this behavious since I can’t think of a reason why the default it preferable. Alternatively I might add a generic class to opt in to this behaviour.

Putting It Into Practice

To implement this solution in your own flexbox layout, follow these steps:

  1. Identify the column(s) where you want to prevent overflowing.
  2. Add the CSS rule min-width:0 to those column(s).

For example, let’s say you have a container with two columns, and you want only the first column to be flexible while preventing it from overflowing:

.container {
	display: flex;

.flexible-column {
	min-width: 0;

By applying min-width:0 specifically to the flexible-column, you can ensure that its content stays within bounds and doesn’t cause any frustrating overflows.

Let me know what you think on Mastodon, or BlueSky (or Twitter X if you must).

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