QQuickWidget activeInputItem is never set if parent is a Widget using Qt::PopUp flag

Scenario:

• a QWidget with the Qt::PopUp WindowFlag is used as a popup across the application
• the popup works fine if it's children are regular QWidgets

• a QQuickWidget is added as a direct child to the custom QWidget-popup
• a Qml file with a single TextInput is loaded into the QQuickWidget
• the popup is displayed to the user using QWidget::show

Expected Behaviour:

• the popup should show
• the QQuickWidget should load and render the Qml file
• the user should be able to enter text inside the TextInput shown in the QQuickWidget

Actual Behaviour:

• the popup shows
• the QQuickWidget loads and renders the Qml file
• the user cannot enter any text inside the TextInput as the keyboard focus is never set inside the QQuickWidget

• the situation remains unchanged even after clicking into the TextInput
• the situation remains unchanged if forceActiveInput is used on the TextInput
• mouse interaction is fully working, but no keyboard input

Questions you might ask:

• why are you using QQuickWidget instead of QQuickWindow?
  because otherwise we couldn't wrap it into our custom popup

• why use a custom QWidget popup and not directly use the QQuickWidget as popup?
  the custom QWidget popup uses Qt::WA_TranslucentBackground and some extra painting to achieve a "balloon" style popup look which is otherwise not available

Additional Information:

Further analysis (using QT_LOGGING_RULES=qt.quick.focus=true) shows that the activeInputItem is never set or changed, even if explicitly ordered to do so by using forceActiveInput in the TextInput or by clicking on the TextInput.