Attempt to use IsAttached to identify if a menu is a menubar menu

We can't get a mouse position for a menubar menu's event, because the
mouse may be outside our canvas. The original hack to detect menubar
menus may not work fully, and there have been some crashes appearing on
macOS where menubar menus attempted to get mouse position information.

Instead of the hack, try using IsAttached() to detect if the menu is
attached to a menubar, and then gate mouse position access accordingly.

Fixes #16738