Click the dock icon to restore after closing the window

Change the file macos/Runner/AppDelegate.swift as follows:

import Cocoa
import FlutterMacOS

@NSApplicationMain
class AppDelegate: FlutterAppDelegate {
    override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
        return false
    }

    override func applicationShouldHandleReopen(_ sender: NSApplication, hasVisibleWindows flag: Bool) -> Bool {
        if !flag {
            for window in NSApp.windows {
                if !window.isVisible {
                    window.setIsVisible(true)
                }
                window.makeKeyAndOrderFront(self)
                NSApp.activate(ignoringOtherApps: true)
            }
        }
        return true
    }
}