Just found one workaround to login Chat Mac App: Just change all the 'IN_NEW_WINDOW' and "EXTERNAL" to "_blank" in /Applications/Chat.app/Contents/Resources/app/main.js. Then start the Chat App and login, you can use the Chat App now.

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toot.Thoughtworks

A Mastodon instance running on Thoughtworks infrastructure for its employees to interact with the Fediverse.