MacOS – Prevent macOS Messages.app from quitting when I close its window

I use a very specific workflow with the Messages app on the Mac:

1. Read my messages
2. Close the Messages window with CMD–W
3. Messages app is still running
4. When a new message comes in, CMD–Tab to the still-open Messages app
5. Press CMD–0 to re-open the Messages window

After closing the Messages window, the app has always remained open and running in the Dock and app switcher.

In Mac OS 10.14 Mojave, I similarly close the Messages window. After closing the Messages window, the Messages app is still in the foreground, running, and shows the menu bar.

When I click to some other app, however, the Messages app then immediately seemingly Quits.

The dot disappears from the dock, and (this is the key issue) I can no longer CMD–Tab to the Messages app in the app switcher.

I'm forced to go click the dock icon with my mouse or otherwise re-open the app, which is inconvenient and annoying.

After the app has Quit, besides being "Quit", it seems to behave more-or-less exactly how it used to behave pre-Mojave. The Messages process is still running and visible in Activity Monitor. Notifications continue to arrive. The dock icon gets badged.

How do I prevent Messages from quitting when I close its window?

rdar://46142132 – Mojave Messages.app should not quit when I close its window

Engineering has determined that your bug report (46142132) is a duplicate of 42085189 and will be closed.

Related links:

• With Mojave (macOS 10.14), why does the Message app close itself?
• Messages app now quits when close window?
  
• Messages app quits on Mojave instead of closing