How I create a "HeaderMenu" with a Popover menu in GTK?
<p>I'm trying to create a "HeaderMenu" like this:</p>

<p><a href="https://i.stack.imgur.com/4ZmsU.png" rel="noreferrer"><img src="https://i.stack.imgur.com/4ZmsU.png" alt="enter image description here"></a></p>

<p>But only I got it:</p>

<p><a href="https://i.stack.imgur.com/weZ9T.png" rel="noreferrer"><img src="https://i.stack.imgur.com/weZ9T.png" alt="enter image description here"></a></p>

<p>I'm using GtkMenuButton within a GtkHeaderBar. How do I get a menu like the first picture?</p>

<p>Codes:</p>

<p><a href="http://paste.debian.net/788667" rel="noreferrer">Glade file</a>
<a href="http://paste.debian.net/788669" rel="noreferrer">Python code</a></p>