Is it possible to have the "Edit source" tab disabled but have the source editor as default just like on Wikipedia?
Topic on Help talk:VisualEditor/User guide
If you mean "here on this wiki", you need to login first. Then, you will be able to choose your favorite Editing mode from a dropdown in https://www.mediawiki.org/wiki/Special:Preferences#mw-prefsection-editing .
I meant for logged out users on my wiki.
I have now added $wgVisualEditorUseSingleEditTab = true; to my LocalSettings.php file which has made it now show a single tab that defaults to edit source which is more or less what I want.
In Special:Preferences#mw-prefsection-editing there's an "Editing mode" preference. This is part of the VisualEditor/Single edit tab. I believe that you can configure your wiki to default to any of the items in that dropdown menu.