On https://patchdemo.wmflabs.org/wikis/014e812f6d/wiki/Talk:New_section (demo for T282204), we noticed errors like this when trying to post a new topic, or viewing the page with topic subscriptions enabled:
[e3c6d34443edfd248060b0ab] /wikis/014e812f6d/wiki/Talk:New_section TypeError: Argument 1 passed to Wikimedia\Parsoid\Utils\DOMCompat::getOuterHTML() must be an instance of Wikimedia\Parsoid\DOM\Compat\Element, instance of DOMElement given, called in /srv/patchdemo-wikis/014e812f6d/w/extensions/DiscussionTools/includes/CommentFormatter.php on line 275
Backtrace:
from /srv/patchdemo-wikis/014e812f6d/w/parsoid/src/Utils/DOMCompat.php(386)
#0 /srv/patchdemo-wikis/014e812f6d/w/extensions/DiscussionTools/includes/CommentFormatter.php(275): Wikimedia\Parsoid\Utils\DOMCompat::getOuterHTML(DOMElement)
#1 [internal function]: MediaWiki\Extension\DiscussionTools\CommentFormatter::MediaWiki\Extension\DiscussionTools\{closure}(array)
#2 /srv/patchdemo-wikis/014e812f6d/w/extensions/DiscussionTools/includes/CommentFormatter.php(231): preg_replace_callback(string, Closure, string)
#3 /srv/patchdemo-wikis/014e812f6d/w/extensions/DiscussionTools/includes/Hooks/PageHooks.php(157): MediaWiki\Extension\DiscussionTools\CommentFormatter::postprocessTopicSubscription(string, LanguageEn, MediaWiki\Extension\DiscussionTools\SubscriptionStore, User)
#4 /srv/patchdemo-wikis/014e812f6d/w/includes/HookContainer/HookContainer.php(160): MediaWiki\Extension\DiscussionTools\Hooks\PageHooks->onOutputPageBeforeHTML(OutputPage, string)
#5 /srv/patchdemo-wikis/014e812f6d/w/includes/HookContainer/HookRunner.php(2686): MediaWiki\HookContainer\HookContainer->run(string, array)
#6 /srv/patchdemo-wikis/014e812f6d/w/includes/OutputPage.php(2017): MediaWiki\HookContainer\HookRunner->onOutputPageBeforeHTML(OutputPage, string)
#7 /srv/patchdemo-wikis/014e812f6d/w/includes/OutputPage.php(2029): OutputPage->addParserOutputText(ParserOutput, array)
#8 /srv/patchdemo-wikis/014e812f6d/w/includes/page/Article.php(843): OutputPage->addParserOutput(ParserOutput, array)
#9 /srv/patchdemo-wikis/014e812f6d/w/includes/page/Article.php(751): Article->doOutputFromRenderStatus(MediaWiki\Revision\RevisionStoreRecord, Status, OutputPage, array)
#10 /srv/patchdemo-wikis/014e812f6d/w/includes/page/Article.php(559): Article->generateContentOutput(User, ParserOptions, integer, OutputPage, array)
#11 /srv/patchdemo-wikis/014e812f6d/w/includes/actions/ViewAction.php(74): Article->view()
#12 /srv/patchdemo-wikis/014e812f6d/w/includes/MediaWiki.php(538): ViewAction->show()
#13 /srv/patchdemo-wikis/014e812f6d/w/includes/MediaWiki.php(320): MediaWiki->performAction(Article, Title)
#14 /srv/patchdemo-wikis/014e812f6d/w/includes/MediaWiki.php(925): MediaWiki->performRequest()
#15 /srv/patchdemo-wikis/014e812f6d/w/includes/MediaWiki.php(559): MediaWiki->main()
#16 /srv/patchdemo-wikis/014e812f6d/w/index.php(53): MediaWiki->run()
#17 /srv/patchdemo-wikis/014e812f6d/w/index.php(46): wfIndexMain()
#18 {main}It looks like commit rGPARbcaca588336b: Add class alias file to allow swapping in Dodo for DOMDocument changed type hints in an incompatible way. The errors will probably soon also occur on beta cluster and production? What should we do to fix this?