The Automatic Minimum Size of Flex Items

You’re encountering a flexbox default setting.

A flex item cannot be smaller than the size of its content along the main axis.

The defaults are…

• min-width: auto
• min-height: auto

…for flex items in row-direction and column-direction, respectively.

You can override these defaults by setting flex items to:

• min-width: 0
• min-height: 0
• overflow: hidden (or any other value, except visible)

Flexbox Specification

4.5. Automatic Minimum Size of Flex
Items

To provide a more reasonable default minimum size for flex items, this
specification introduces a new auto value as the initial value of
the min-width and min-height properties defined in CSS 2.1.

With regard to the auto value…

On a flex item whose overflow is visible in the main axis, when specified on the flex item’s main-axis min-size property, specifies an automatic minimum size. It otherwise computes to 0.

In other words:

• The min-width: auto and min-height: auto defaults apply only when overflow is visible.
• If the overflow value is not visible, the value of the min-size property is 0.
• Hence, overflow: hidden can be a substitute for min-width: 0 and min-height: 0.

and…

• The minimum sizing algorithm applies only on the main axis.
• For example, a flex item in a row-direction container does not get min-height: auto by default.
• For a more detailed explanation see this post:
  • min-width rendering differently in flex-direction: row and flex-direction: column

You’ve applied min-width: 0 and the item still doesn’t shrink?

Nested Flex Containers

If you’re dealing with flex items on multiple levels of the HTML structure, it may be necessary to override the default min-width: auto / min-height: auto on items at higher levels.

Basically, a higher level flex item with min-width: auto can prevent shrinking on items nested below with min-width: 0.

Examples:

• Flex item is not shrinking smaller than its content
• Fitting child into parent
• white-space css property is creating issues with flex

Browser Rendering Notes

• Chrome vs. Firefox / Edge

  Since at least 2017, it appears that Chrome is either (1) reverting back to the min-width: 0 / min-height: 0 defaults, or (2) automatically applying the 0 defaults in certain situations based on a mystery algorithm. (This could be what they call an intervention.) As a result, many people are seeing their layout (especially desired scrollbars) work as expected in Chrome, but not in Firefox / Edge. This issue is covered in more detail here: flex-shrink discrepancy between Firefox and Chrome

• IE11

  As noted in the spec, the auto value for the min-width and min-height properties is “new”. This means that some browsers may still render a 0 value by default, because they implemented flex layout before the value was updated and because 0 is the initial value for min-width and min-height in CSS 2.1. One such browser is IE11. Other browsers have updated to the newer auto value as defined in the flexbox spec.

Revised Demo

.container {
  display: flex;
}

.col {
  min-height: 200px;
  padding: 30px;
  word-break: break-word
}

.col1 {
  flex: 1;
  background: orange;
  font-size: 80px;
  min-width: 0;   /* NEW */
}

.col2 {
  flex: 3;
  background: yellow
}

.col3 {
  flex: 4;
  background: skyblue
}

.col4 {
  flex: 4;
  background: red
}

<div class="container">
  <div class="col col1">Lorem ipsum dolor</div>
  <div class="col col2">Lorem ipsum dolor</div>
  <div class="col col3">Lorem ipsum dolor</div>
  <div class="col col4">Lorem ipsum dolor</div>
</div>

jsFiddle

I’m finding this has bitten me repeatedly over the years for both flex and grid, so I’m going to suggest the following:

* { min-width: 0; min-height: 0; }

and then just use min-width: auto or min-height: auto if you need that behaviour.

In fact, throw in box-sizing as well to make all layout more sane:

* { box-sizing: border-box; min-width: 0; min-height: 0; }

Does anyone know if there are any odd consequences? I’ve not encountered anything in several years of using a mix of the above. In fact, I can’t think of any cases where I’d want to layout from content outwards to the flex/grid, rather than flex/grid inwards to the content — and surely if they exist, they’re rare. So this feels like a bad default. But maybe I’m missing something?

For this code below, adding width: 100% solved my problem.

.post-cover .inner {
    display: flex;
    flex-direction: column;
    flex-wrap: wrap;
    justify-content: center;
    align-content: flex-start;
    align-items: flex-start;
    word-break: break-all;
    z-index: 21;
}
.post-cover .article-page {
    padding: 20px 0;
    margin-bottom: 40px;
    font-size: 0.875em;
    line-height: 2.0;
    overflow: hidden;
    text-overflow: ellipsis;
    white-space: nowrap;
    width: 100%;   /* Add this */
}

The pure answer to your question is that by default, browsers tend to display as much information as possible to the reader (and not to hide anything).

That happens by default, and even includes showing default black color fonts on a white background (for maximum page contrast and readability), adding a scroll bar where content is larger than the viewport height (or width) or still showing content from a markup (or the background color) even if this was mistakenly placed after </body> or even </html> tags in the html file.

In context of CSS, this applies as well, but you also are allowed to play with many customizations on top of that.

Even in a screen if using a huge font (like font-size: 50em;) this initially acts as an overflowing element (and placing the font inside a flexible child container by using display: flex doesn’t change this default behaviour unless you use overflow: hidden or resize the element in some way.

An elegant solution is to use a dynamic resizing of the letters, for example

font-size: calc(0.5em + 2vw)

which works great even in a responsive scenario.

As a previous answer mentioned, A flex item cannot be smaller than the size of its content along the main axis (for the same reason, that is not only specific to the flexbox model implemented in CSS but because of the inner browser way of working). Even a long word is displayed with a scrollbar if it’s longer than display width as if being a block type element with a fixed size instead.

This is mentioned in old html 4.01 specifications as

“By convention, visual HTML user agents wrap text lines to fit within
the available margins. Wrapping algorithms depend on the script being
formatted.

In Western scripts, for example, text should only be wrapped at white
space. “

as seen here in paragraph 9.5.3. This means that, since then, the text had to be continuously displayed by default (unless we decide to split it but not at single character level: a single non-white character shown at 120em size will trigger scrollbars displaying on the browser).

Words are also clearly defined in paragraph 9.1 in the same source:

we use the term “word” here to mean “sequences of non-white space
characters”

The purpose of displaying the original format of any word is to not destroy, hide or distort the original information, the meaning or intent of the code author. As such, we also have   for keeping in same line two words that are connected – when breaking them might be disruptive (such as New York, 10 PM, 10 km/h, § 10, etc)

What about:

.parent {
  display: flex;
  width: 420px;
}

.child-with-ellipses {
  flex-basis: auto;
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
}

.child-without-ellipses {
  flex-basis: 69%;
  background-color: red;
}

<div class="parent">
  <div class="child-with-ellipses">
    This node has the class `child-with-ellipses`.
  </div>
  <div class="child-without-ellipses"></div>
</div>

The most efficient solution as for now is using -webkit-line-clamp: https://developer.mozilla.org/en-US/docs/Web/CSS/-webkit-line-clamp

You can specify number of lines to preserve, for example 1.

Old solution based on min-width: 0 does not work on complex layouts with wide usage of flex.

I tried everything, even putting the below code in the index.css.

* {
  min-width: 0;
  min-height: 0;
  box-sizing: border-box;
}

But nothing worked.

finally I made the div I wanted to shrink past it’s content to have position: absolute;. Then it started shrinking.
It’s parent div would need a defined height and width. This might not be the best solution for every scenario but if this works for you, good!