Q:

Rails 5 + Boptstrap 4: Navbar not working on mobile

This is my first stackoverflow question..
I'm trying to install Boostrap 4 on Rails 5, I thought that everything was working but when I checked the website from mobile I noticed that the navbar is really small compared to the "usual" size (but at least it's correctly collapsed). 
I followed the official "Bootstrap Ruby Gem"but it doesn't work. 
This is the current website with the incriminated navbar (it's just the basic navbar from boostrap components):
https://warm-peak-79199.herokuapp.com/
Following my files:
GemFile:
gem 'bootstrap', '~> 4.0.0.beta2.1'

Application.scss:
// Custom bootstrap variables must be set or imported *before* bootstrap. 
@import "bootstrap";

Application.js:
// This is a manifest file that'll be compiled into application.js, which will include all the files
// listed below. 
//
// Any JavaScript/Coffee file within this directory, lib/assets/javascripts, or any plugin's
// vendor/assets/javascripts directory can be referenced here using a relative path. 
//
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// compiled file.  JavaScript code in this file should be added after the last require_* statement. 
//
// Read Sprockets README (https://github.com/rails/sprockets#sprockets-directives) for details
// about supported directives. 
//
//= require rails-ujs
//= require turbolinks
//= require jquery3
//= require popper
//= require bootstrap-sprockets
//= require_tree . 

Application.erb.html:
<! DOCTYPE html>
  <html>
    <head>
      <title>FlickerFeed</title>
      <%= csrf_meta_tags %>

      <%= stylesheet_link_tag    'application', media: 'all', 'data-turbolinks-track': 'reload' %>
      <%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
    </head>

    <body>
      <nav class="navbar navbar-expand-lg navbar-light bg-light">
        <a class="navbar-brand" href="#">Navbar</a>
        <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>

        <div class="collapse navbar-collapse" id="navbarSupportedContent">
          <ul class="navbar-nav mr-auto">
            <li class="nav-item active">
              <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
            </li>
            <li class="nav-item">
              <a class="nav-link" href="#">Link</a>
            </li>
            <li class="nav-item dropdown">
              <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
                Dropdown
              </a>
              <div class="dropdown-menu" aria-labelledby="navbarDropdown">
                <a class="dropdown-item" href="#">Action</a>
                <a class="dropdown-item" href="#">Another action</a>
                <div class="dropdown-divider"></div>
                <a class="dropdown-item" href="#">Something else here</a>
              </div>
            </li>
            <li class="nav-item">
              <a class="nav-link disabled" href="#">Disabled</a>
            </li>
          </ul>
          <form class="form-inline my-2 my-lg-0">
            <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
            <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
          </form>
        </div>
      </nav>
    </body>
  </html>

Can anyone help me understanding what's wrong? 
Thank you so much! 
Isashi

A:

Your site is missing the responsive meta tag.   Add this to your layout inside the <head> tags:
<meta name="viewport" content="width=device-width, initial-scale=1.0">

