In wikitext, `[[foo]]bar` will turn into `<a href="foo">foobar</a>` assuming `bar` matches the [[https://en.wikipedia.org/wiki/MediaWiki:Linktrail | linktrail regexp ]]. VisualEditor instead offers a visual cue: when one is about to type a character which will be part of the link, the link is highlighted, otherwise it's not (and moving the cursor towards/away from the link can be used to switch between those states).
This is intuitive and easy to use and would be neat interaction design, except that the two actions are not symmetric: unlinked characters at the end of a link are basically never used. So VisualEditor makes it very easy to create misformatted text (which looks extra weird in the source, too: T127635), while with the old editor it's practically impossible.
VisualEditor should automatically make linktrail characters part of the link as they are typed (in the ultra-rare case of not wanting that, the user can enter the trailing part as a separate word and then backspace it onto the link), or some kind of warning should be shown when one adds unlinked trailing characters.