import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.5

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

    Dialog {
        id: dialog

//        width: 320            //Works
        width: parent.width / 2 //Do not works

        modal: true

        closePolicy: Popup.NoAutoClose

        footer: DialogButtonBox {
            Button {
                text: qsTr("Close")
                DialogButtonBox.buttonRole: DialogButtonBox.RejectRole
            }
            Button {
                text: qsTr("Open")
                DialogButtonBox.buttonRole: DialogButtonBox.AcceptRole
            }
        }
    }

    MouseArea {
        anchors.fill: parent

        onClicked: dialog.open()
    }
}