TYPO3 Core

In a Composer-based setup, the root composer.json package is listed as extension.

This behavior is triggered by having extra->typo3/cms section defined (not matter which properties exactly) like the following:

    "extra": {
        "typo3/cms": {
            "web-dir": "public" 
        }
    },

Removing this section completely from the root composer.json (web-dir: public is the default behavior) removes the listing in extension manager as well.