This patch reduces paint events by removing code which sets native
widgets dirty in QWidgetWindow::handleExposeEvent. Native widgets are
also marked dirty in QWidgetPrivate::drawWidget, so it is enough for
proper painting.
This restores Qt4 behavior when one resize means one repaint for native
widgets. Without this patch the native widget is marked as dirty on
every expose event, so one repaint is from syncBackingStore and second
(or more) is from marking the widget dirty explicitly.
This patch improves performance of native widgets and it also reduces
locks when paint event is v-synced, e.g. on OpenGL swap buffers or on
any other technology like VDPAU, VA-API, etc.
Added autotest for checking number of paint events for native widgets.
Task-number: QTBUG-50796
Change-Id: I4e1649069e2e73d15b038fd1834d0551915252ee
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
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