JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <div class="messages">
    <div class="message">1</div>
    <div class="message">2</div>
    <div class="message">3</div>
    <div class="message">4</div>
    <div class="message">5</div>
    <div class="message">6</div>
    <div class="message">7</div>
    <div class="message">8</div>
    <div class="message">9</div>
    <div class="message">10</div>
    <div class="message">11</div>
    <div class="message">12</div>
    <div class="message">13</div>
    <div class="message">14</div>
    <div class="message">15</div>
    <div class="message">16</div>
    <div class="message">17</div>
    <div class="message">18</div>
    <div class="message">19</div>
    <div class="message">20</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")
}