As long as the surface type of the child window is the same as the one owning the backingstore we can flush it through the same code path as for the top level window. This code path was already exercised by QWidgetRepaintManager::flush(), by calling rhiFlush() directly, but we now support it in those (rare) cases of non-Widgets windows using a single QBackingStore for a full window hierarchy. A future improvement would be to decouple the RHI configuration from the backingstore, so that we can flush child windows with different surface types through the same backingstore. This is relevant for platforms that rely on QRhiBackingStore exclusively (iOS and Android), as in widgets we currently only switch the top level widget surface type when enabling RHI based flushing, leaving all child windows with QWindow::RasterSurface. This could potentially be solved by teaching widgets to re-configure all QWidgetWindows, including children. Task-number: QTBUG-119309 Change-Id: Ifa3fe6125493ac1becd2538ba84f3b4a5e3a5d71 Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io> Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>