Hello @SGaist
That's correct. Sadly, in my use case usual way of closing the context menu by click outside of its bounds has side effects, as in close neighborhood of the menu there are many QWidgets whose react to mouse clicks. Hence the need of dedicated close/cancel button.

However, I've finally found a hint in Assistant "If the QWidgetAction fires the triggered() signal, the menu will close.".
So, the problem is solved for my case, the solution is just:

connect(closeButton, &QPushButton::clicked, menuTitle, &QWidgetAction::triggered);

but thank you mate nonetheless for trying to help.

@Roysten-Rigobert Why don't you create menu/actions at once by making menu as member variable?

and in contextMenu(QPoint) slot call only the menu->exec(mapToGlobal(pos));

Thank you for your detailed answer. I know where to go from here.