macOS: Ensure foreign windows can be reparented via QWindow::setParent()

In b64b0c7947 we bailed out from
QCocoaWindow::recreateWindowIfNeeded() for foreign windows, as
we should not manage any NSWindows on their behalf.

Unfortunately QCocoaWindow::recreateWindowIfNeeded() also took care
of adding the view as a subview to the potential non top level view,
which we do want for foreign views.

Ideally we'd move the reparenting out of recreateWindowIfNeeded()
and into QCocoaWindow::setParent(), but this is a more intrusive
change, so for now just restore the original missing logic.

Pick-to: 6.6 6.5
Change-Id: Ic35ebf94d4adc2f19cedb1cb6a5d0215a1c9c2b4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -1448,16 +1448,22 @@ void QCocoaWindow::recreateWindowIfNeeded()
 {
     QMacAutoReleasePool pool;
 
+    QPlatformWindow *parentWindow = QPlatformWindow::parent();
+    auto *parentCocoaWindow = static_cast<QCocoaWindow *>(parentWindow);
+
     if (isForeignWindow()) {
         // A foreign window is created as such, and can never move between being
         // foreign and not, so we don't need to get rid of any existing NSWindows,
         // nor create new ones, as a foreign window is a single simple NSView.
         qCDebug(lcQpaWindow) << "Skipping NSWindow management for foreign window" << this;
+
+        // We do however need to manage the parent relationship
+        if (parentCocoaWindow)
+            [parentCocoaWindow->m_view addSubview:m_view];
+
         return;
     }
 
-    QPlatformWindow *parentWindow = QPlatformWindow::parent();
-
     const bool isEmbeddedView = isEmbedded();
     RecreationReasons recreateReason = RecreationNotNeeded;
 
@@ -1495,8 +1501,6 @@ void QCocoaWindow::recreateWindowIfNeeded()
     if (recreateReason == RecreationNotNeeded)
         return;
 
-    QCocoaWindow *parentCocoaWindow = static_cast<QCocoaWindow *>(parentWindow);
-
     // Remove current window (if any)
     if ((isContentView() && !shouldBeContentView) || (recreateReason & PanelChanged)) {
         if (m_nsWindow) {