I know, we should all stop using IE11 with prejudice, now that it's out of extended support. Feel free to close this as "won't fix". Just noting it for the record.
Where seen: fr-wp with ?dtenable=1
Observed behaviour: click the Reply link, Loading... message appears and stays, text-entry box does not appear.
(Aside: I had set my UI language to en, so I do see "Reply" and "Loading..." rather than "Répondre" and "Chargement en cours...".)
We probably want to support the same browsers as MediaWiki, including IE 11, unless is proves impractical to do so.
Change 576094 had a related patch set uploaded (by Bartosz Dziewoński; owner: Bartosz Dziewoński):
[mediawiki/extensions/DiscussionTools@master] modifier: Fix IE 11 incompatibility due to 'parentElement'
Change 576094 merged by jenkins-bot:
[mediawiki/extensions/DiscussionTools@master] modifier: Fix IE 11 incompatibility due to 'parentElement'
I hope y'all have some Windows machines in the office… ;) If not, I tested it and can confirm it works with this patch.
Moving to sign-off based on @matmarex's comment. Don't have any Windows machine with me to double check this.