JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="messages">
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
    <div class="message">hello</div>
  </div>
  <div class="send-message">
    <input />
  </div>
</div>

<button onclick="test()">add margin</button>

CSS

.container {
  width: 400px;
  height: 300px;
  border: 1px solid #333;
  display: flex;
  flex-direction: column;
}

.messages {
  overflow-y: auto;
  height: 100%;
}

.send-message {
  width: 100%;
  display: flex;
  flex-direction: column;
}

.some-margin {
  margin-bottom: 100px;
}

JavaScript

window.onload = function(e){ 
    document.querySelector(".messages").scrollTop = 10000;
}

function test() {
	document.querySelector(".send-message").classList.toggle("some-margin")
}