We should remove or fix the config option to have both mobile DiscussionTools and MobileFrontend talk page interface enabled at the same time. (`$wgDiscussionToolsEnableMobile = 'behind-overlay'`)
It is currently enabled on the beta cluster (e.g. https://en.m.wikipedia.beta.wmflabs.org/wiki/Talk:Editnotice_test). This is because Editing team wanted to test mobile DiscussionTools on the beta cluster for {T282638}, but when we made changes to enable it and disable the MobileFrontend interface, this caused failures in browser tests there and general annoyance because {T293946} is in progress and Reading team wanted to be able to test that on the beta cluster too.
The option to have both was thus hastily implemented in <https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DiscussionTools/+/738012>. Naturally this turns out to be buggy (the "Add discussion" button opens both interfaces at once, not sure if there are any other bugs).
If we're going to use that option in production, we'll need to find and fix the bugs. @Esanders insists that we might not use it, and that we should remove it instead. (https://gerrit.wikimedia.org/r/c/mediawiki/extensions/DiscussionTools/+/738226)