The Qt::WindowState enum is documented to specify the current state of top-level windows, and does not make sense for child windows. It's up to the client code to ensure any sizing of child windows/widgets, e.g. via layouts. Many of our QPA backends bail out on QPlatformWindow::setWindowState() for a child window. Ideally we would do this at a higher level, in QWindow, but that's a bigger task, and the semantics of what happens when a window is moved from being top level to child or back are not fully clear. As a first step, we ensure that the default window state when showing the window via show() is Qt::WindowNoState. Unfortunately, the QPlatformIntegration::defaultWindowState() API only takes into account the window flags, which sadly do not have a way to reflect whether the window is a child window or not (Qt::SubWindow is not a child window, see QTBUG-115729). We don't want to pass a QWindow to this API, as it would mean QWidget would need to create a window when requesting the default window state. Instead we hard-code the opt-out for child windows/widgets. [ChangeLog][Gui/Widgets] Child windows and widgets are now always shown in their normal state by show(). Pick-to: 6.7 Change-Id: Ie8caf2df2d854194b9985c9427a9eac1312aba65 Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>