QWindow: prevent window reparenting into Qt::Desktop windows

Qt::Desktop exists to support QDesktopWidget, which predates the QScreen
API. QWidget internally has checks that prevents you from reparenting a
QWidget into a QDesktopWidget, so we should have the same limitations on
the QWindow level. This allows platform plugins to implement Qt::Desktop
as simple (possibly shared) wrappers around QScreen without having to
allocate native window resources for each desktop window.

Change-Id: Ia1bac506febd3d827a6e0b8ad3bfd95be0cc7f9d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qwindow.cpp

@@ -155,8 +155,17 @@ QWindow::QWindow(QScreen *targetScreen)
     , QSurface(QSurface::Window)
 {
     Q_D(QWindow);
-    d->connectToScreen(targetScreen ? targetScreen : QGuiApplication::primaryScreen());
-    d->init();
+    d->init(targetScreen);
+}
+
+static QWindow *nonDesktopParent(QWindow *parent)
+{
+    if (parent && parent->type() == Qt::Desktop) {
+        qWarning("QWindows can not be reparented into desktop windows");
+        return nullptr;
+    }
+
+    return parent;
 }
 
 /*!
@@ -170,14 +179,8 @@ QWindow::QWindow(QScreen *targetScreen)
     \sa setParent()
 */
 QWindow::QWindow(QWindow *parent)
-    : QObject(*new QWindowPrivate(), parent)
-    , QSurface(QSurface::Window)
+    : QWindow(*new QWindowPrivate(), parent)
 {
-    Q_D(QWindow);
-    d->parentWindow = parent;
-    if (!parent)
-        d->connectToScreen(QGuiApplication::primaryScreen());
-    d->init();
 }
 
 /*!
@@ -193,13 +196,10 @@ QWindow::QWindow(QWindow *parent)
     \sa setParent()
 */
 QWindow::QWindow(QWindowPrivate &dd, QWindow *parent)
-    : QObject(dd, parent)
+    : QObject(dd, nonDesktopParent(parent))
     , QSurface(QSurface::Window)
 {
     Q_D(QWindow);
-    d->parentWindow = parent;
-    if (!parent)
-        d->connectToScreen(QGuiApplication::primaryScreen());
     d->init();
 }
 
@@ -215,10 +215,15 @@ QWindow::~QWindow()
         QGuiApplicationPrivate::instance()->modalWindowList.removeOne(this);
 }
 
-void QWindowPrivate::init()
+void QWindowPrivate::init(QScreen *targetScreen)
 {
     Q_Q(QWindow);
 
+    parentWindow = static_cast<QWindow *>(q->QObject::parent());
+
+    if (!parentWindow)
+        connectToScreen(targetScreen ? targetScreen : QGuiApplication::primaryScreen());
+
     // If your application aborts here, you are probably creating a QWindow
     // before the screen list is populated.
     if (Q_UNLIKELY(!parentWindow && !topLevelScreen)) {
@@ -671,6 +676,8 @@ QWindow *QWindow::parent() const
 */
 void QWindow::setParent(QWindow *parent)
 {
+    parent = nonDesktopParent(parent);
+
     Q_D(QWindow);
     if (d->parentWindow == parent)
         return;

src/gui/kernel/qwindow_p.h

@@ -113,7 +113,7 @@ public:
     {
     }
 
-    void init();
+    void init(QScreen *targetScreen = nullptr);
 
     void maybeQuitOnLastWindowClosed();
 #ifndef QT_NO_CURSOR