QWidgetPrivate::updateIsTranslucent sets the surface format of the
window with the alpha based on the translucency attribute, so we need
to call this function when the attribute value changes. The test can
confirm that the window's requested surface format has changed, we
can't rely on what is actually set, and don't have to rely on
hard-coded values like 8bit alpha.
While WA_NoSystemBackground needs to be set for WA_TranslucentBackground
to have an effect, we can't clear the attribute when clearing
translucency (as it might have been set explicitly).
Change-Id: I238d6930b7e0488397467a4e035b5f530566a1ff
Fixes: QTBUG-60822
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
555661b625