Adapted version of https://codepen.io/chriscoyier/pen/XWKEVLy - see also https://chriscoyier.net/2023/09/29/css-solves-auto-expanding-textareas-probably-eventually/

grow-wrap.css

.grow-wrap {
  /* easy way to plop the elements on top of each other and have them both sized based on the tallest one's height */
  display: grid;
}
.grow-wrap::after {
  /* Note the weird space! Needed to preventy jumpy behavior */
  content: attr(data-replicated-value) " ";

  /* This is how textarea text behaves */
  white-space: pre-wrap;

  /* Hidden from view, clicks, and screen readers */
  visibility: hidden;
}
.grow-wrap > textarea {
  /* You could leave this, but after a user resizes, then it ruins the auto sizing */
  resize: none;

  /* Firefox shows scrollbar on growth, you can hide like this. */
  overflow: hidden;
}
.grow-wrap > textarea,
.grow-wrap::after {
  /* Identical styling required!! */
  border: 1px solid black;
  border-radius: 3px;
  padding: 0.35rem;
  font: inherit;
  line-height: 1.4;
  font-family: sans-serif;

  /* Place on top of each other */
  grid-area: 1 / 1 / 2 / 2;
}

body {
  margin: 2rem;
  font: 1rem/1.4 system-ui, sans-serif;
}

label {
  display: block;
}

grow-wrap.html

<h1>Auto-Growing <code>&lt;textarea></code></h1>

<form action="#0">

  <label for="text">Text:</label>
  <div class="grow-wrap">
    <textarea rows="1" name="text" placeholder="Placeholder shows here" id="text" onInput="this.parentNode.dataset.replicatedValue = this.value"></textarea>
  </div>

</form>

Marvelous. @simonw if I were to adapt and use this for a project, can I assume it's either covered by your TIL Apache 2.0 license, or unlicensed?

FYI I had to add word-break: break-word; to .grow-wrap::after