"Username" vs. "User name"

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I stumbled about Gerrit:52151. My first thought was to patch "User name" -> "Username" as is seems to be the default at least in core. But a search throughout a lot of extensions shows me a big mix. What should be the default: "Username" or "User name"? Is this a typical AE vs. BE? Or what?

I checked the Wikipedia page on w:en:User. It says: "A user often has a user account and is identified by a username (also user name)"

That totally does it: We choose "username"!