I initially started with this ticket by implementing a new Special page as it was done in the ThankYou extension. However, I then talked to Thiemo and he pointed me to a more suitable example and how this would be better suited to be implemented as an action which checks if the request was made as a GET or a POST to determine if the form is to be shown or if the rollback action is to be performed.

A similar action would be the purge action which can be triggered via ?action=purge on articles, for example:

https://de.wikipedia.org/wiki/Stummelschwanz-Zwergtyrann?action=purge

The relevant action can be found at:

mediawiki/includes/actions/PurgeAction.php

Change 491235 had a related patch set uploaded (by Tim Eulitz; owner: Tim Eulitz):
[mediawiki/core@master] Implement non-JS RollbackAction with form

https://gerrit.wikimedia.org/r/491235

Change 491235 abandoned by Tim Eulitz:
Implement non-JS RollbackAction with form

Reason:
https://gerrit.wikimedia.org/r/c/mediawiki/core/ /491249

https://gerrit.wikimedia.org/r/491235

While I'm reviewing this, I generally wonder if there should be a Cancel button/link in the form? Usually(™) there's something like this, right? ( Sorry if this was already discussed and I don't know and for bringing it up generally :-D )

We discussed this, and thanks does not have one. So staying consistent we are not having one either :) (Especially since you can cancel by using the back button, closing the tab or not doing anything)