Return the actual modal window in QGuiApplicationPrivate::isWindowBlocked

The method erroneously returns the common ancestor of the |window| in
question and a modal window in case of window-modality of the modal
window. What should be returned is the modal window because it is the
one that blocks the |window|.

Fixes: QTBUG-106627
Change-Id: Ib4da6c1e9757629bbedbd8a16407cd6b5b0d5d27
Pick-to: 6.4
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qguiapplication.cpp

@@ -899,7 +899,7 @@ bool QGuiApplicationPrivate::isWindowBlocked(QWindow *window, QWindow **blocking
             auto *current = window;
             do {
                 if (current->isAncestorOf(modalWindow, QWindow::IncludeTransients)) {
-                    *blockingWindow = current;
+                    *blockingWindow = modalWindow;
                     return true;
                 }
                 current = current->parent(QWindow::IncludeTransients);