nested mdi: Fix detaching inner dock widgets

Only the outter-most MDI frame is dragged in MDI mode. The inner ones
are dragged in normal docking mode, they become real floating windows.

src/private/TitleBar.cpp

@@ -101,7 +101,26 @@ MainWindowBase *TitleBar::mainWindow() const
 
 bool TitleBar::isMDI() const
 {
-    return firstParentOfType<MDILayoutWidget>(this) != nullptr;
+    QObject *p = const_cast<TitleBar *>(this);
+    while (p) {
+        if (qobject_cast<const QWindow*>(p)) {
+            // Ignore QObject hierarchies spanning though multiple windows
+            return false;
+        }
+
+        if (qobject_cast<MDILayoutWidget *>(p))
+            return true;
+
+        if (qobject_cast<DropArea *>(p)) {
+            // Note that the TitleBar can be inside a DropArea that's inside a MDIArea
+            // so we need this additional check
+            return false;
+        }
+
+        p = p->parent();
+    }
+
+    return false;
 }
 
 void TitleBar::updateButtons()

tests/tst_docks.cpp

@@ -5095,12 +5095,12 @@ void TestDocks::tst_mdi_mixed_with_docking2()
 
     Frame *frame1 = mdiWidget1->d->frame();
     Frame *mdiFrame1 = frame1->mdiFrame();
-    QPointer<DropArea> dropArea1 = frame1->mdiDropAreaWrapper();
 
     QPointer<Frame> frame2 = mdiWidget2->d->frame();
     QPointer<Frame> mdiFrame2 = frame2->mdiFrame();
     QPointer<DropArea> dropArea2 = frame2->mdiDropAreaWrapper();
 
+    QPointer<DropArea> dropArea1 = frame1->mdiDropAreaWrapper();
     dropArea1->addDockWidget(mdiWidget3, Location_OnLeft, nullptr);
 
     QVERIFY(!frame1->isMDI());
@@ -5191,6 +5191,21 @@ void TestDocks::tst_mdi_mixed_with_docking2()
 
     QVERIFY(Testing::waitForDeleted(mdiFrame1));
     QCOMPARE(mdiArea->frames().size(), 0);
+
+    // Dock again:
+    mdiArea->addDockWidget(mdiWidget1, QPoint(10, 10));
+    mdiArea->addDockWidget(mdiWidget2, QPoint(50, 50));
+
+    frame1 = mdiWidget1->d->frame();
+    dropArea1 = frame1->mdiDropAreaWrapper();
+    dropArea1->addDockWidget(mdiWidget3, Location_OnLeft, nullptr);
+
+
+    // Detach an internal dock widget by dragging
+    const QPoint globalSrc = mdiWidget1->mapToGlobal(QPoint(5, 5));
+    const QPoint globalDest = globalSrc + QPoint(10, 0);
+    drag(mdiWidget1, globalDest);
+
     // QTest::qWait(100000);
 }