Use Qt::WindowNoState for child windows/widgets shown via show()

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().

Change-Id: Ie8caf2df2d854194b9985c9427a9eac1312aba65
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
(cherry picked from commit 3472aa6e28469ffd23cfa24e3933541f295e9e1d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qwindow.cpp

@@ -2231,20 +2231,26 @@ QObject *QWindow::focusObject() const
 /*!
     Shows the window.
 
-    This is equivalent to calling showFullScreen(), showMaximized(), or showNormal(),
+    For child windows, this is equivalent to calling showNormal().
+    Otherwise, it is equivalent to calling showFullScreen(), showMaximized(), or showNormal(),
     depending on the platform's default behavior for the window type and flags.
 
     \sa showFullScreen(), showMaximized(), showNormal(), hide(), QStyleHints::showIsFullScreen(), flags()
 */
 void QWindow::show()
 {
-    Qt::WindowState defaultState = QGuiApplicationPrivate::platformIntegration()->defaultWindowState(d_func()->windowFlags);
+    if (parent()) {
+        showNormal();
+    } else {
+        const auto *platformIntegration = QGuiApplicationPrivate::platformIntegration();
+        Qt::WindowState defaultState = platformIntegration->defaultWindowState(d_func()->windowFlags);
         if (defaultState == Qt::WindowFullScreen)
             showFullScreen();
         else if (defaultState == Qt::WindowMaximized)
             showMaximized();
         else
             showNormal();
+    }
 }
 
 /*!

src/widgets/kernel/qwidget.cpp

@@ -7954,21 +7954,29 @@ void QWidget::setUpdatesEnabled(bool enable)
 /*!
     Shows the widget and its child widgets.
 
-    This is equivalent to calling showFullScreen(), showMaximized(), or setVisible(true),
+    For child windows, this is equivalent to calling setVisible(true).
-    depending on the platform's default behavior for the window flags.
+    Otherwise, it is equivalent to calling showFullScreen(), showMaximized(),
+    or setVisible(true), depending on the platform's default behavior for the window flags.
 
     \sa raise(), showEvent(), hide(), setVisible(), showMinimized(), showMaximized(),
     showNormal(), isVisible(), windowFlags()
 */
 void QWidget::show()
 {
-    Qt::WindowState defaultState = QGuiApplicationPrivate::platformIntegration()->defaultWindowState(data->window_flags);
+    // Note: We don't call showNormal() as not to clobber Qt::Window(Max/Min)imized
+
+    if (!isWindow()) {
+        setVisible(true);
+    } else {
+        const auto *platformIntegration = QGuiApplicationPrivate::platformIntegration();
+        Qt::WindowState defaultState = platformIntegration->defaultWindowState(data->window_flags);
         if (defaultState == Qt::WindowFullScreen)
             showFullScreen();
         else if (defaultState == Qt::WindowMaximized)
             showMaximized();
         else
-        setVisible(true); // Don't call showNormal() as not to clobber Qt::Window(Max/Min)imized
+            setVisible(true);
+    }
 }
 
 /*! \internal