How can I make my app behave like I'm changing Application is agent(UIElement) at runtime using Swift?

It took a lot of trial and error, but I finally figured it out. Instead of using Application is agent(UIElement), you use NSApp.setActivationPolicy. This is now my code. In the App Delegate:

var isWindow = false

class AppDelegate: NSObject, NSApplicationDelegate, NSPopoverDelegate {

    let statusItem = NSStatusBar.system.statusItem(withLength: NSStatusItem.squareLength)
    let popover = NSPopover()

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        // Insert code here to initialize your application

        NSApp.setActivationPolicy(.accessory)

        if let button = statusItem.button {
            button.image = NSImage(named: "StatusBarImage")
            button.action = #selector(togglePopover(_:))
        }
        popover.contentViewController = MainViewController.loadController()
        popover.delegate = self
        popover.animates = false
        popover.behavior = .transient
    }

    @objc func togglePopover(_ sender: Any?) {
        if popover.isShown == true {
            popover.performClose(sender)
        } else if detachedWindowController.window!.isVisible {
            detachedWindowController.window?.setIsVisible(false)
            isWindow = true
        } else if isWindow == true {
            detachedWindowController.window?.setIsVisible(true)
        } else {
            if let button = statusItem.button {
                popover.show(relativeTo: button.bounds, of: button, preferredEdge: NSRectEdge.minY)
            }
        }
    }

    lazy var detachedWindowController: DetachedWindowController = {
        let detachedWindowController = DetachedWindowController(windowNibName: "DetachedWindowController")
        detachedWindowController.contentViewController = MainViewController.loadController()
        return detachedWindowController
    }()

    func popoverShouldDetach(_ popover: NSPopover) -> Bool {
        return true
    }

    func detachableWindow(for popover: NSPopover) -> NSWindow? {
        return detachedWindowController.window
    }
}

In the DetachedWindowController:

class DetachedWindowController: NSWindowController, NSWindowDelegate {

    @IBOutlet var detachedWindow: NSWindow!

    override func windowDidLoad() {
        super.windowDidLoad()

        detachedWindow.delegate = self
    }

    func windowWillClose(_ notification: Notification) {
        isWindow = false
        NSApp.setActivationPolicy(.accessory)
    }

    func windowDidBecomeMain(_ notification: Notification) {
        if NSApp.activationPolicy() == .accessory {
            NSApp.setActivationPolicy(.regular)
        }
    }
}