unexpected anchors in ApplicationWindow

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.4

ApplicationWindow {
    id: rootWindow
    
    visible: true
    width: 400
    height: 400
    title: qsTr("Hello World")
    
    header: ToolBar {}
    
    Popup {
        id: popup
        
        anchors.centerIn: parent
        
        width: 300
        height: 300
        
        
        modal: true
        padding: 0
        
        Rectangle {
            anchors.fill: parent
            color: "red"
        }
        
        Component.onCompleted: open()
    }
}

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.4

ApplicationWindow {
    id: rootWindow
    
    visible: true
    width: 400
    height: 400
    title: qsTr("Hello World")
    
    header: ToolBar {}
    
    Popup {
        id: popup
        
        anchors.centerIn: parent
        
        width: 300
        height: rootWindow.height
        
        
        modal: true
        padding: 0
        
        Rectangle {
            anchors.fill: parent
            color: "red"
        }
        
        Component.onCompleted: open()
    }
}