mastodon-bigger-input-box.user.js

// ==UserScript==
// @name         mastodon-bigger-input-box
// @version      1.0.0
// @description  Modify the CSS of a specific element on page load
// @author       Thomas Reggi
// @match        https://indieweb.social/*
// @grant        none
// ==/UserScript==

(function() {
  'use strict';
  const targetSelector = "#mastodon > div > div > div.columns-area__panels > div.columns-area__panels__pane.columns-area__panels__pane--compositional > div > div > form > div.compose-form__highlightable > div.compose-form__autosuggest-wrapper > div.autosuggest-textarea > label > textarea";
  const targetElement = document.querySelector(targetSelector);
  const applyStyles = (targetElement) => {
      targetElement.style.setProperty('height', '500px', 'important');
      targetElement.style.setProperty('max-height', '40vh', 'important');
  };

  const observeChanges = () => {
      // Setting up the mutation observer
      const observer = new MutationObserver((mutations) => {
          for (const mutation of mutations) {
              if (mutation.type === 'attributes' && mutation.attributeName === 'style') {
                  applyStyles(targetElement);
              }
          }
      });

      // Starting the observation
      observer.observe(targetElement, {
          attributes: true,
          attributeFilter: ['style']
      });
  };
  applyStyles(targetElement);
  observeChanges()

})();