title: Why is the button not expanding to fill all the space?
body: <p>I've made a tiny test GtkD DUB project (with GtkD 3.9.0 as dependency) to make a minimal reproducible example of a problem I am having in a larger project. Apparently Box is not expanding the middle Widget (in this case Button) to take all available space. Am I doing something wrong?</p>
  
  <pre class="lang-d prettyprint-override"><code>import gtk.MainWindow;
  import gtk.Label;
  import gtk.Button;
  import gtk.Main;
  import gtk.Box;
  
  void main(string[] args) {
      Main.init(args);
  
      MainWindow window = new MainWindow(&quot;Pack test&quot;);
      Box box = new Box(Orientation.VERTICAL, 0);
      Label topLabel = new Label(&quot;Top&quot;);
      box.add(topLabel);
      box.packStart(topLabel, false, false, 0);
      Button bigButton = new Button(&quot;Big button&quot;);
      box.add(bigButton);
      box.packStart(bigButton, true, true, 0);
      Label bottomLabel = new Label(&quot;Bottom&quot;);
      box.add(bottomLabel);
      box.packStart(bottomLabel, false, false, 0);
      window.add(box);
      window.setDefaultSize(800, 600);
      window.showAll();
  
      Main.run();
  }
  </code></pre>
  <p>When I make the same structure in Glade and go to preview, it shows the button expanded to fill all available space, so either I am doing something wrong, or there is a bug in GtkD...</p>