JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.css">
<script src="http://code.jquery.com/jquery-2.1.4.min.js"></script>
<script src="http://cdn.leafletjs.com/leaflet/v0.7.7/leaflet.js"></script>
<div id="map">
  <div id="map-nav"></div>
  <div id="map-content"></div>
</div>

CSS

#map {
  width: 100%;
  height: 300px;
  position: relative;
}
#map-nav {
  position: absolute;
  left: 10px;
  top: 10px;
  width: 100px;
  height: 100px;
  background-color: #eee;
  z-index: 2;
}
#map-content {
  width: 100%;
  height: 100%;
}
.leaflet-popup-pane, .leaflet-popup {
  z-index: 1000 !important;
}

JavaScript

var map = L.map('map-content', {
	zoomControl: false
}).setView([51.505, -0.09], 13);
L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token={accessToken}', {
    attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, <a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, Imagery © <a href="http://mapbox.com">Mapbox</a>',
    maxZoom: 18,
    id: 'rauschundrausch.cih3gisks00r8rlly0ob5s2me',
    accessToken: 'pk.eyJ1IjoicmF1c2NodW5kcmF1c2NoIiwiYSI6ImNpaTYyeW14MTAwNml2eW0zcmR6aHIzdDcifQ.6T8nzYq49rFnvcqk6lgYPg'
}).addTo(map);

var marker = L.marker([51.5, -0.09]).addTo(map);
marker.bindPopup("this is a<br>large<br>popup<br>in<br>terms of height");