Steam will not exit gracefully on macOS 10.12 Sierra. When selecting exit from the application menu or via secondary click on the app icon in the dock nothing happens. The process has to be terminated forcefully, and it will also stop the OS's poweroff process. On occasion the program will exit properly but I cannot see any pattern to it.
When running the application from the command line and then attempting to exit, at first nothing appears in the output, but after a minute or so the following is logged:
KQ: Error Unregistering write event KQ:Operation now in progress Setting breakpad minidump AppID = 769 Steam_SetMinidumpSteamID: Caching Steam ID: 76561198003850785 [API loaded no]
macOS Sierra 10.12.4, Steam API: v017, Steam package versions: 1490228413