[Malleys]

dropdown-menu сделать на весь экран

Добавьте сл. стили в свой проект... (Если нужно, уточните селектор)

.dropdown-menu {
	position: fixed !important;
	transform: none !important;
	top: 0; left: 0;
	right: 0; bottom: 0;
	width: 100%; height: 100%;
	margin: 0; border: 0;
	border-radius: 0;
	overflow: auto;
}

Пример...

<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css">
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"></script>

<div class="dropdown">
	<button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
    Dropdown button
  </button>
	<div class="dropdown-menu" aria-labelledby="dropdownMenuButton">
		<a class="dropdown-item" href="#">Action</a>
		<a class="dropdown-item" href="#">Another action</a>
		<a class="dropdown-item" href="#">Something else here</a>
	</div>
</div>

<style>

html, body {
	display: grid;
	place-content: center;
	height: 100%;
	margin: 0;
}

.dropdown-menu {
	position: fixed !important;
	transform: none !important;
	top: 0; left: 0;
	right: 0; bottom: 0;
	width: 100%; height: 100%;
	margin: 0; border: 0;
	border-radius: 0;
	overflow: auto;
}
	
</style>