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HTML 

              
                <header>
      <h1>overscroll-behavior demo</h1>
    </header>
    <main>

    </main>
    <section class="chatbox">
      <header>
        <h2>Active chat</h2>
      </header>
      <div class="messages">
        <p class="me">Chris: Hello!</p>
        <p class="you">Bob: I am well — how are you?</p>
        <p class="me">Chris: Fine thanks, just documenting overscroll-behavior</p>
        <p class="you">Bob: Oooh, sounds hard!</p>
        <p class="me">Chris: Nah, it's OK really. It is a useful property to have for mobile apps especially.</p>
        <p class="you">Bob: Cool, where can I learn more?</p>
      </div>
      <form>
        <input type="text" aria-label="Enter your chat message here" autofocus>
        <button>Send</button>
      </form>
    </section>
    <footer>
      <p>Copyright nobody; inspired by <a href="https://ebidel.github.io/demos/chatbox.html">ebidel's chatbox demo</a>.</p>
    </footer>
!

CSS 

              
                /* General set up */

* {
  box-sizing: border-box;
}

html {
  font-family: sans-serif;
}

body {
  margin: 0;
  /* Use overscroll-behavior to stop the "bounce" or refresh effect you get on mobile
  browsers when getting to the top or bottom scroll limits of the page */
  overscroll-behavior: none;
}

/* Styling of contacts */

header, footer {
  background-color: #a60000;
  color: white;
  text-align: center;
  height: 50px;
  line-height: 50px;
  text-shadow: 2px 2px 1px black;
}

h1, h2 {
  margin: 0;
}

main div {
  padding: 10px 20px;
  border-bottom: 1px solid #eee;
}

p {
  margin: 0;
}

a {
  color: #ccc;
}

main div p {
  line-height: 20px;
}

body > header {
  position: sticky;
  top: 0;
}

/* Styling of chatbox */

.chatbox {
  background: white;
  width: 200px;
  height: 300px;
  border: 1px solid black;
  position: fixed;
  bottom: 10px;
  right: 10px;
}

.messages {
  height: 220px;
  overflow: auto;
  /* use overscroll-behavior-y to stop page scrolling behind the chatbox,
  when the chatbox's scroll limits are reached */
  overscroll-behavior-y: contain;
}

.messages p {
  margin: 10px;
  padding: 10px;
  border-radius: 3px;
}

.me {
  background-color: #bbb;
}

.you {
  background-color: #eee;
  text-align: right;
}

.chatbox form {
  height: 30px;
  display: flex;
}

form input, form button {
  display: block;
}

form input {
  width: 70%;
}

form button {
  width: 30%;
  font-size: 0.7rem;
}
!

JS 

              
                // Generate fake contacts

let mainElem = document.querySelector('main');

for(var i = 1; i < 51; i++) {
  let divElem = document.createElement('div');
  let pElem = document.createElement('p');

  pElem.textContent = 'Contact ' + i;

  mainElem.appendChild(divElem);
  divElem.appendChild(pElem);
}

// Allow submission of chat messages

let form = document.querySelector('form');
let input = document.querySelector('input');
let messages = document.querySelector('.messages');
messages.scrollTop = messages.scrollHeight;

form.addEventListener('submit', e => {
  e.preventDefault();

  if(input.value !== '') {
    let pElem = document.createElement('p');
    pElem.setAttribute('class', 'me');
    pElem.textContent = 'Chris: ' + input.value;

    messages.appendChild(pElem);
    messages.scrollTop = messages.scrollHeight;

    input.value = '';
    input.focus();
  }
});
!
999px

Console