Cannot drop into WindowContainer

When using WindowContainer to embed a foreign window, it is not possible to drop into the foreign window.

This can be tested by using the draggable icons example as a foreign window,
jut add qDebug() << mainWidget.winId(); to get it's win id before app.exec().

Then in a QML app:

engine.rootContext()->setContextProperty("externalWindow", QWindow::fromWinId(<winId from dragable icons example>));

import QtQuick

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    Rectangle{
      anchors.fill:parent
      color: 'red'

      WindowContainer{
        anchors.fill:parent
        anchors.margins:8
        window: externalWindow
      }
    }
}

Steps

1. Run the draggable icons example
2. Check that drag and drop works fine
3. Note the winid from the console output
4. Copy and paste the winid into the QML app
5. Compile and run the QML app
6. The draggable icons window should be embedded
7. Drag and drop does not work