For anyone else annoyed by this, I've created a userscipt to auto expand the textarea. It's not perfect, but it's better than nothing.

// ==UserScript==
// @name        Autoexpand textarea
// @namespace   Violentmonkey Scripts
// @match       https://p.feddit.uk/*
// @grant       none
// @version     1.0
// @author      flamingos-cant
// @description 06/08/2024, 21:05:31
// ==/UserScript==

function updateHeight(textarea) {
  var text = textarea.target.value;
  var lineHeight = 1.25 * parseFloat(getComputedStyle(document.documentElement).fontSize);
  var taHeight = textarea.target.scrollHeight - 16 * 2;
  var lines = Math.max(Math.ceil(taHeight / lineHeight), 8);
  textarea.target.rows = lines;
}

setInterval((function () {
  let textareas = document.getElementsByTagName("textarea");
  for (let textarea of textareas) {
    textarea.addEventListener("input", updateHeight);
  }
}), 2000);

@Emperor@feddit.uk I'll update photon soon to increase the size of the text box as you type.

I'll also tweak parts of the UI to make nested scrolling places more obvious. You can always mention me about this stuff, I want to keep this UI intuitive!