title: Minimize and maximize buttons don't work but maximizing/minimizing by alt+space menu works
body: <p>I have a window as shown below, the max/min buttons don't work but I can use the <kbd>alt</kbd>+<kbd>space</kbd> menu to max/min/resize the window:</p>
  
  <p><a href="https://i.stack.imgur.com/l6cyv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/l6cyv.png" alt="max/min/resize issue"></a></p>
  
  <p>The window is a <code>QMainWindow</code>:</p>
  
  <pre><code>// MainWindow constructor
  MainWindow::MainWindow(): QMainWindow(0) {
      // ...
  }
  
  int main(int argc, char *argv[])
  {
      int result;
      try {
          QApplication a(argc, argv);
          MainWindow w;
          w.show();
          result = a.exec();
      }
      catch (...) {
          result = -1;
      }
      return result;
  }
  </code></pre>
  
  <p>I'm on openSUSE Leap 15 with Gnome 3:</p>
  
  <pre><code>&gt; gnome-shell --version
  GNOME Shell 3.26.2
  </code></pre>
  
  <hr>
  
  <p>I'm using <code>Qt 5.11</code></p>
  
  <hr>
  
  <p>I'm not setting any size policy for <code>MainWindow</code>. Did anybody run into this issue before?</p>