Looking at the code in https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FEcho/6c8e3a0c3a7a32bee87fcf89f78370286bd38d50/includes%2FDiscussionParser.php#L714 this actually should work, and https://de.wikipedia.org/w/index.php?title=Benutzer_Diskussion%3ASchnark&diff=138274729&oldid=137934707 did work, so there must be some other reason why it didn't work. The spans are wrongly nested, but this too wasn't a problem in https://de.wikipedia.org/w/index.php?title=Benutzer_Diskussion%3ASchnark&diff=138274875&oldid=138274729. Therefore I'll merge it as duplicate of the mysterious T78424.

 if ( !$winningUser ) {
    return false;
}

Shouldn't that be

  if ( $winningUser === false ) {
    return false;
}

$startCheckAt = max( 0, $timestampPos - 300 );
$context = substr( $potentialContext, -$startCheckAt );

is this correct?

@FriedhelmW That particular bit of code was just changed today, mostly just to add rational for how it works and why at https://gerrit.wikimedia.org/r/#/c/173455/5/includes/DiscussionParser.php

Your best bet to determine if it is correct is to work through the unit tests for that particular function. The test for that function is at https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/tests/phpunit/includes/DiscussionParserTest.php#L62 . It is fed data from the dataProvider a few lines down, the signingDetectionData() method. For a given message the function needs to return two pieces of data, The number of characters in the content and the name of the user that signed it. It has become a bit odd over time trying to fit all the conditions set forth by real world cases that have made it into that unit test.

@FriedhelmW also @matthiasmullie just submitted https://gerrit.wikimedia.org/r/#/c/187850/ today, which is making a much bigger overhaul. If you wanted to test and code review the patch it would be most appreciated.