The old stack structure used to keep track of windows has been improved
to conform to the actual windowing assumptions: there shall be one root
window, which is always at the bottom. The first created window
immediately becomes the root window. Should the root window be removed,
all windows are non-root, i.e. any of them can become the top-level window
Fixes: QTBUG-105094
Pick-to: 6.4
Change-Id: Ic553244fa9f5bc3ee590b702935e66cfc62d5f8f
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
1007964f2d