NSPopover - Hide when focus lost? (clicked outside of popover)

You need to change the property behavior of your popover (in code or on interface builder) to:

popover.behavior = NSPopover.Behavior.transient;

NSPopover.Behavior.transient
The system will close the popover when the user interacts with a user interface element outside the popover.

Read more about this in Apple's documentation.

the .transient flag doesn't work for me.

However I can make things work by the following:

1) Whenever I show my popover I make sure I activate the app (my app is a menu-bar app, so this doesn't happen automatically)

NSApp.activate(ignoringOtherApps: true)

2) When I click outside the app, then my app will be deactivated. I can detect this in the AppDelegate

func applicationWillResignActive(_ notification: Notification) {
    print("resign active")
}

and act accordingly