QQuickAttachedPropertyPropagator cannot propagate through popup and window, which is different from the behavior of palette(for example).

It looks like that it works fine in the demo attachedstyleproperties, but when we move the custom Popup and the ApplicationWindow (defined in MyStyle) into an Item which is a child of the main ApplicationWindow,

ApplicationWindow {
      ......
     color: _test.MyStyle.windowColor //avoid to set MyStyle.theme
     Item {
            id: _test
        anchors.fill: parent
        MyStyle.theme: darkModeSwitch.checked ? MyStyle.Dark : MyStyle.Light //set here
            ......
         Popup { ......
        ApplicationWindow { ......
    }
} 

the Popup and ApplicationWindow will fail to follow the MyStyle of their parent, while the two Openxxx buttons and labels can.

There maybe a bug that propagated attached properties of Popup and ApplicationWindow will follow the value of the Popup and AppcationWindow's window, not their parent.

AttachedParent of Popup's attached property is the Window's attached property, not their parent's