WikiEditor has a method "addView" which sets up a extra tab, with the default being "Wikitext". This is not used anywhere, and possibly never has been. This code was for a now-deleted feature that was never used in production (see T380034#10378008).
Example:
$('#wpTextbox1').data('wikiEditor-context').fn.addView({name:'new', title: 'New'})
Codesearch: https://codesearch.wmcloud.org/search/?q=addView&files=&excludeFiles=&repos=
Global search: https://global-search.toolforge.org/?q=%5C.addView%5C%28®ex=1&namespaces=2%2C4%2C8&title=%28Gadgets-definition%7C.*%5C.%28js%7Ccss%7Cjson%29%29
