Observer: <nowiki/> is added immediately after the closing double square bracket.
For an example see https://en.wikipedia.org/wiki/User:Amire80/halflink .
Isn't this by design, though? If you write [[wh]]en in wikitext, the link appears for the entire word "when" as linked to 'wh'. In VisualEditor, if you, on purpose, only select a part of a word, we assume you only want that part of the word to be the link and the only way to do that in wikitext is to add nowiki.
Indeed. This is performing exactly what the user asked for. It's also the cause of many "accidental" no-wikis, but the alternative is over-riding user requests and instead doing what we think they should do, which is confusing and so we have avoided doing much of this throughout VisualEditor.