Issue closing parent window during QMenu contextmenu exec on macOS

A non-modal QDialog contextmenu does not grab the mouse click focus on
macOS when being opened unlike Linux and Windows

This allows a user to leave an open contextmenu and close the MainWindow
by hitting the x on the MainWindow title bar, resulting in a crash

To recreate this bug on macOS:

unzip bug.zip
cd bug
qmake
make
open ./bug.app

In the smaller QDialog, launch its contextmenu by left clicking.
While the menu is *open*, move mouse and hit the red X in the
MainWindow titlebar. The app will crash.

See:

https://www.qtcentre.org/threads/65046-closing-parent-widget-during-QMenu-exec()

link title Dated: Jan 2016

The macOS platform should act just like Windows and Linux here and grab mouse click
focus so that any mouse click on the titlebar of the MainWindow close X will simply close
the open contextmenu.

The only workaround is to use a QPointer<QMenu> here to detect that
the menu was actually already destroyed.

As far as I know this bug exists in all version of Qt since January 2016 timeframe.

Attached a standalone test case for macOS. Same test case works just fine on Linux and Windows.