import QtQuick import QtQuick.Controls // ApplicationWindow can't be resolved because it's in QtQuick.Controls ... ApplicationWindow { // ... therefore we don't know whether "header" is a Component. // ... which means we cannot tell whether item1 is inside that "Component" or outside of it. header: Item { property Item item1: item1 } Item { objectName: "outer" id: item1 } objectName: header.item1.objectName }