JSFiddle - React, Tailwind, and code Playground

HTML

<main>
  <dialog>
    <header>
      Example
    </header>
    <div class="content">
      <form>
        <div class="custom-drop-down-element">
          <input />
          <ul class="dropdown-list">
            <li>Example 1</li>
            <li>Example 2</li>
          </ul>
        </div>
      </form>
    </div>
    <footer>
      <button>
        Close
      </button>
    </footer>
  </dialog>
</main>

CSS

* {
  box-sizing: border-box;
}

main {
  background: white;
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
  position: relative;
}

dialog {
  position: fixed;
  top: 50%;
  left: 0; right: 0;
  margin: 0 auto;
  transform: translateY(-50%);
  background: white;
  border: 1px solid black;
  width: 400px;
  height: auto;
  display: block;
  padding: 0;
}

header {
  width: 100%;
  border-bottom: 1px solid black;
  padding: 10px;
}

footer {
  width: 100%;
  border-top: 1px solid black;
  padding: 10px;
}

.content {
  max-height: 400px;
  overflow-y: auto;
  width: 100%;
  padding: 10px;
}

.custom-drop-down-element {
  width: 100%;
  position: relative;
}

.dropdown-list {
  width: 100%;
  position: absolute;
  top: 100%;
  left: 0; right: 0;
}