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HTML 

              
                <main>
  <div class="wrapper">
    <article class="flow">
      <figure class="callout">
        <p>Use the inputs to modify the <code>overflow</code> property.</p>
      </figure>
      <div class="cluster">
        <div class="controls">
          <div>
            <label for="switcher">overflow:</label>
            <select id="switcher">
              <option value="scroll">scroll</option>
              <option value="visible">visible</option>
              <option value="hidden">hidden</option>
              <option value="clip">clip</option>
              <option value="auto">auto</option>
              <option value="hidden auto">hidden auto</option>
            </select>
          </div>
        </div>
        <div class="chat-ui">
          <div class="chat-titlebar">
            <h5>Chat UI</h5>
              <div class="chat-avatar">
                <img src="https://cdn.dribbble.com/users/37530/screenshots/2937858/drib_blink_bot.gif" alt="">
              </div>
          </div>
          <div class="chat-scrollview">
    <div class="chat-messagelist">
      
      <div class="chat-cluster">
        <div class="chat-avatar">
          <img src="https://cdn.dribbble.com/users/37530/screenshots/2937858/drib_blink_bot.gif" alt="">
        </div>
        <section>
          <h3>Learn CSS Bot</h3>
          <div class="chat-message">Hi!</div>
          <div class="chat-message">I'm a bot</div>        
        </section>
      </div>
      
      <div mine class="chat-cluster">
        <section>
          <div class="chat-message">Hi!</div>
          <div class="chat-message">I'm not a bot</div>
          <div class="chat-message">But I am static text that was put here by a human. The only dynamic stuff you'll find here is whatever someone types in the "say something" text area hehe.</div>
        </section>
      </div>
      
      <div class="chat-cluster">
        <div class="chat-avatar">
          <img src="https://cdn.dribbble.com/users/37530/screenshots/2937858/drib_blink_bot.gif" alt="">
        </div>
        <section>
          <h3>Learn CSS Bot</h3>
          <div class="chat-message">Makes sense.</div>
          <div class="chat-message">I see this chat implementation is using the overflow property...</div>
          <div class="chat-message">Can you tell me more about that?</div>
        </section>
      </div>
      
      <div mine class="chat-cluster">
        <section>
          <div class="chat-message">sure!</div>
          <div class="chat-message">Element `overflow` is a property set on an element to control what happens when the children of an element are requesting more space than it has available.</div>
          <div class="chat-message">Learn CSS can teach you more about overflow!</div>
        </section>
      </div>
      
    </div>
  </div>
  
          <div class="chat-authoring" contenteditable></div>
        </div>
      </div>
    </article>
  </div>
</main>
!

CSS 

              
                :root {
  --var-overflow: 'scroll';
}

.chat-ui, .chat-scrollview {
  overflow: var(--var-overflow);
}

.controls {
  display: inline-grid;
  grid-template-columns: minmax(150px, max-content) 1fr;
  gap: 0 1rem;
  align-items: center;
}
!

JS 

              
                let root = document.documentElement;
let overflow= document.getElementById("switcher");
overflow.addEventListener("change", function (evt) {
  root.style.setProperty('--var-overflow', evt.target.value);
});

root.style.setProperty('--var-overflow', 'scroll');
!
999px

Console