Allow dragging of a floating dockwidget on macOS with a custom titlebar

This amends 3224c6d7d1 to account for the
case when the dockwidget is already floating.

Task-number: QTBUG-70137
Pick-to: 6.2 6.1 5.15
Change-Id: If8b345565b11b44beb3fb4b697cfe812c29c6396
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/widgets/qdockwidget.cpp

@@ -974,7 +974,7 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
                 > QApplication::startDragDistance()) {
 
 #ifdef Q_OS_MACOS
-            if (windowHandle()) {
+            if (windowHandle() && !q->isFloating()) {
                 // When using native widgets on mac, we have not yet been successful in
                 // starting a drag on an NSView that belongs to one window (QMainWindow),
                 // but continue the drag on another (QDockWidget). This is what happens if