Setting a cancel button on QProgressDialog more than once caused the layout
to be invalid. The layout was only applied when the dialog resizes or the
style changes, but not when a new cancel button is set.
The solution is to update the layout() before showing the dialog when adopting
new child widgets.
Fixes: QTBUG-19983
Pick-to: 6.0 6.1
Change-Id: Id8fb1ac56e94a9bd97d4559a2e8d4835856fd7d0
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
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