From https://once.com/writebook

app/views/leaves/_header.html.erb

@@ -0,0 +1,26 @@
+<% content_for :header do %>
+  <%= leaf_nav_tag(leaf) do %>
+    <%= link_to_previous_leafable(leaf) %>
+
+    <div class="breadcrumbs">
+      <%= render "books/index_link" %>
+      <span class="flex-item-no-shrink">▸</span>
+      <%= link_to book.title, book_slug_path(book) %>
+      <span class="flex-item-no-shrink">▸</span>
+      <strong><%= leaf.leafable.title %></strong>
+    </div>
+
+    <button class="btn" data-action="fullscreen#toggle" data-fullscreen-target="button">
+      <%= image_tag "expand.svg", aria: { hidden: true }, size: 24 %>
+      <span class="for-screen-reader">Enter fullscreen</span>
+    </button>
+  <% end %>
+<% end %>
+
+<% content_for :toolbar do %>
+  <% if book.editable? %>
+    <div class="page-toolbar fill-shade">
+      <%= editing_mode_toggle_switch(@leaf, checked: false) %>
+    </div>
+  <% end %>
+<% end %>