for a menu you want this:

+        const Qt::Edge sideEdge = q->isRightToLeft()  ? Qt::LeftEdge : Qt::RightEdge;
+        q->windowHandle()->setProperty("_q_waylandPopupAnchor", QVariant::fromValue<Qt::Edges>(Qt::TopEdge | sideEdge));
+        q->windowHandle()->setProperty("_q_waylandPopupGravity", QVariant::fromValue<Qt::Edges>(Qt::BottomEdge | sideEdge));