Qt Quick borderless window not redrawn - qt

Question

I try to create a simple application where the user needs to enter a date. For this I want to connect a button with a calendar popup. This works fine as long as the popup is its own window and has a border. But the moment I make the window borderless it seems to be not drawn anymore. Actually if you move the underlaying window around you see that the area with the calendar widget stays undrawn.
import QtQuick 2.2
import QtQuick.Controls 1.2
import QtQuick.Layouts 1.1
import QtQuick.Window 2.1
ApplicationWindow {
width: 800
height: 600
property var pos : getOffset(dateField)
Button {
id: dateField
width: 100
height: 50
anchors.centerIn: parent
text: "show widget"
onClicked:{ toggleModal() }
}
function getOffset(item) {
var offset = {
"x": item.x,
"y": item.y + item.height
};
while(item.parent) {
item = item.parent;
offset.x += item.x;
offset.y += item.y;
}
console.debug("total", "x", offset.x, "y", offset.y)
return offset;
}
function toggleModal() {
if(modal.active) {
console.log("hide calendar")
loseFocus();
}
else {
console.log("show calendar")
modal.show()
modal.requestActivate()
}
}
function loseFocus(newDate) {
modal.close();
}
Window {
id: modal
flags: Qt.Window | Qt.FramelessWindowHint
//flags: Qt.Window
//modality: Qt.ApplicationModal
minimumHeight: calendar.height; minimumWidth: calendar.width
maximumHeight: calendar.height; maximumWidth: calendar.width
x: pos.x
y: pos.y
Calendar {
id: calendar
width: 200
height: 300
}
}
}