The proper fix for subwindow parenting issues.

The previous patch tried to work around the fact that Qt::SubWindow
accidentally pulled in Qt::Window. This patch corrects the root
cause of the problem.

src/corelib/global/qnamespace.h

@@ -278,7 +278,7 @@ public:
         ToolTip = Popup | Sheet,
         SplashScreen = ToolTip | Dialog,
         Desktop = 0x00000010 | Window,
-        SubWindow = Desktop | Dialog,
+        SubWindow = 0x00000012,
 
         WindowType_Mask = 0x000000ff,
         MSWindowsFixedSizeDialogHint = 0x00000100,

src/widgets/kernel/qwidget_qpa.cpp

@@ -96,6 +96,12 @@ void QWidgetPrivate::create_sys(WId window, bool initializeWindow, bool destroyO
     QWindowSurface *surface = q->windowSurface();
 
     QWindow *win = topData()->window;
+    // topData() ensures the extra is created but does not ensure 'window' is non-null
+    // in case the extra was already valid.
+    if (!win) {
+        createTLSysExtra();
+        win = topData()->window;
+    }
 
     win->setWindowFlags(data.window_flags);
     win->setGeometry(q->geometry());