Please avoid adding project tags as subscribers - thanks.

It took me a bit to understand what the user is talking about. The bracket matching works fine. It's the red-ish background shading that is broken for the very last }}:

In other words, the bug is in the "mediawiki" mode a.k.a. the syntax highlighter for wikitext. I was able to minimize the counterexample like this:

{{#if: | {{some template
<!-- comment --> }} }}

The bug is triggered by the newline at the end of the template name. Technically, newlines in template names are not allowed. That's why the syntax highlighter kind of "gives up" at this point. But the actual wikitext parser does not care about this specific newline. It's not in the template name, but after. The parser strips comments and trailing newlines, and works just fine with the remaining "some template" template name.

It should be possible to update the syntax highlighter so it's a little more relaxed in a situation like this.

Conclusion from discussions during story time:

This is an existing issue (prior to our work on the syntax highlighter), that seems to be caused by HTML comments. The fix may be straightforward, so we will do a time-boxed exploration to try to solve it. If it turns out to be more complicated, we'll reassess before investing more effort.

Change 682128 had a related patch set uploaded (by Thiemo Kreuz (WMDE); author: Thiemo Kreuz (WMDE)):

[mediawiki/extensions/CodeMirror@master] Allow HTML comments and newlines in template names

https://gerrit.wikimedia.org/r/682128

Change 682128 merged by jenkins-bot:

[mediawiki/extensions/CodeMirror@master] Allow HTML comments and newlines in template names

https://gerrit.wikimedia.org/r/682128