The Qt Quick Window attached property for an Item is initially null. It only starts reflecting a Window once the Item is placed into a Window. This is problematic for logic that tries to distinguish foo: null from foo: bar.Window.window, for example the Window.transientParent property.

This is not a problem if assigning a Window { id: bar } directly as foo: bar

Nor is it a problem for APIs that allow you to assign an Item to a "window" property by implicitly using the Item as a stable reference to the window.