How to set the maximum width of a column in CSS Grid Layout?

Question

What I want to achieve:

Using CSS Grid Layout, to have a page with a right column which size is derived from its content, but only up to 20% of the window width.

How I thought it would work:

It looks good, but then when I remove the content of the second div, the left column does not collapse:

div {
  border-style: solid;
}

#container {
  width: 300px;
  height: 300px;
  display: grid;
  grid-template-columns: 1fr minmax(auto, 20%);
}

<div id="container">
  <div>
    some content
  </div>
  <div></div>
</div>

My question:

I was under the impression that since minmax()

(...) defines a size range greater than or equal to min and less than or equal to max.

it would mean that in my case the width is set from auto (= 0 when the div is empty) to 20%. It however stays at 20%. Why is it so?

Answer 1

You've misunderstood minmax function. It first tries to apply maximum value and when that's not possible, it applies minimum.

So to fix your layout, you just need to calculate 20% of your container width, apply it using max-width property for you grid item, and use auto in your grid-template-columns property definition for the second column. Demo:

div {
  outline: 1px dotted gray;
}

.container {
  width: 300px;
  height: 300px;
  display: grid;
  grid-template-columns: 1fr auto;
}

.container > :nth-child(2) {
  max-width: 60px; /* 20% x 300px */
}

<div class="container">
  <div>
    some content
  </div>
  <div>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec cursus eu leo ac ultrices. Vivamus ornare, orci sed pretium sollicitudin
  </div>
</div>

<div class="container">
  <div>
    some content
  </div>
  <div></div>
</div>

---

Update: More flexible solution will be to use fit-content function from this answer.