@Physikerwelt I hope this will be implemented some time, since I REALLY need those for the wikibook on General Topology. Of the additions I requested, this may well be the most important one. Is there perhaps a colleague of yours who can take care of the issue? Are there any incentives I may be able to provide to such a colleague? We can discuss the issue via email of course.
@Mathmensch if you really need that the only option I currently see is to implement that by yourself or to convince someone to implement it for you. If there is a pull request, I will take care of the rest. The source code of the grammar component is available from https://github.com/wikimedia/texvcjs and a skilled nodejs programmer with a good understanding of a parser (e.g., from the computer science course on how to build compilers) should be able to implement it in two days.
@Physikerwelt It can't be too complicated. Is there any different reason why you refuse any cooperation on that matter? I mean, you neither know me, and nor do you know anything about me, and the same basically applies to all of Wikimedia. I don't see why I should be met with anger and unwillingness.
I think the problem is T188879. Of course nobody wants to fix texvc: It is piece of software which does not serve any meaningful purpouse anymore, was already outdated 9 years ago and it is probably easier to rewrite it from scratch than to fix all the bugs. Mathmensch is different from the normal people, because they have largely given up reporting bugs like this. (Not sure if "adding middle command" is a bug, but as far as I know it has always been part of amsmath and I don't know any other math display engine where it is not supported)
I don't see where @Physikerwelt approaches you in that way. He is a volunteer who is the only person taking care of Math and he is being realistic about his ability to assist in solving this problem. Math does not have paid resourcing at this time, so all work is being done by volunteers. As human resourcing is the most limited resource we have throughout the movement, the best way to get something done, is always to do it yourself or at least be the main driver in getting/finding someone else to do it. That is not unwillingness, it is the reality of a project that is grounded in volunteering.
@Mathmensch: https://www.mediawiki.org/wiki/Professional_development_and_consulting in case that helps.
(Thanks to @TheDJ for explaining the misunderstanding in maintenance expectations.)
The \middle command does have a fairly good workaround, manually specifying the size using \big|, \Big|, \bigg| or \Bigg| for increasing sizes. Other bugs are harder, people are working to remove the textvc limitation but its hard
I've added an example to https://en.wikibooks.org/wiki/User:Mathmensch/Playground