title: Qml ComboBox with TextField in Popup
body: <p>I have created a custom <code>ComboBox</code> that uses a <code>ListView</code> with a <code>TextField</code> in the footer, that's used to dynamically add options to the <code>ComboBox</code>.
  The problem is, that as soon as the popup loses focus (so when the <code>TextField</code> receives focus), the popup gets closed.
  I tried to force the popup to stay open, which does work, but then prevents the <code>TextField</code> from receiving focus (I guess because the popup regains focus as soon as <code>open()</code> is called).</p>
  
  <pre><code>ComboBox {
    // ...
    popup: Popup {
      property bool forceOpen: false
      onClosed: {
        if(forceOpen)
          open()
      }
  
      contentItem: ListView {
        // ...
        footer: TextField {
          onPressed: forceOpen = true
        }
      }
    }
  }
  </code></pre>
  
  <p>I also tried all values for the <code>closePolicy</code> property of the <code>Popup</code>, but none of them helped.</p>
  
  <p>I am using <strong>Qt5.11</strong>. The <code>forceOpen</code> solution used to work with Qt 5.10, but does not anymore.</p>