When the top level window that a QWindowContainer is in is about to
be destroyed the QWindowContainer must reparent the contained window
into a dummy window, as otherwise the destruction of the top level
will bring down the contained window as well.
We were notifying the window container about this situation when
the window container was moved from being a top level itself, to
being a child widget, but did not have any logic for other ways
the window container could lose its parent QWindow.
An example of this was when RHI-needs would result in recreating
the top revel with a different RHI backend.
We now have a last minute call to toplevelAboutToBeDestroyed in
QWidgetPrivate::deleteTLSysExtra().
Fixes: QTBUG-126303
Pick-to: 6.7 6.5
Change-Id: I5b14e156956ae76a8f53cac31904eaadee9e791f
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 006cbf658ea1f5986bbe1baafa7c146780320661)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
f0f9cc602f