Fix addDockWidgetToContainingWindow with nested main windows

It needs to be added to the direct parent, not to the top-level
main window.

Fixes issue #386

Changelog

@@ -21,6 +21,7 @@
   - Fixed double-click on titlebar not floating it if auto-hide flag was enabled
   - Added Config::setLayoutSpacing(int)
   - LayoutSaverOption::Skip now honours docked dock widgets
+  - Fixed DockWidget::addDockWidgetToContainingWindow() when using nested main windows (#386)
 
 * v2.0.0 (05 December 2023)
   - Architecture rewrite to support more frontends besides QtWidgets and QtQuick.

src/core/DockWidget.cpp

@@ -152,9 +152,9 @@ void DockWidget::addDockWidgetToContainingWindow(DockWidget *other, Location loc
     if (!other)
         return;
 
-    if (auto mainWindow = view()->rootView()->asMainWindowController()) {
+    if (auto mw = mainWindow()) {
         // It's inside a main window. Simply use the main window API.
-        mainWindow->addDockWidget(other, location, relativeTo, initialOption);
+        mw->addDockWidget(other, location, relativeTo, initialOption);
         return;
     }
 

tests/qtwidgets/tst_qtwidgets.cpp

@@ -2805,8 +2805,6 @@ void TestQtWidgets::tst_addDockWidgetToContainingWindowNested()
 
         QCOMPARE(d1->dockWidget()->mainWindow(), m1.get());
         QCOMPARE(d2->dockWidget()->mainWindow(), nestedMainWindow.get());
-
-        QEXPECT_FAIL("", "Bug #386, to be fixed", Continue);
         QCOMPARE(d3->dockWidget()->mainWindow(), nestedMainWindow.get());
     }
 }