Flow was supplying an empty string as a parameter to a message, due to a bug 59198. What surprised me is the rendered HTML turned into a preformatted (typewriter-style) block. This happens because the 'flow-topic-participants' message goes through parse(), and in wikitext a leading blank formats the text as a <pre> tag. You can see this using eval.php:

echo wfMessage( 'flow-topic-participants', 2, 2, '', 'second person'

)->parse(); (note the empty string for first participant); this prints

<pre>and second person </pre>

1) Is there a way to turn off this over-aggressive parsing? It's unwanted for an i18n string you know should just be a span.

2) Is there an idiom to guard against this unintended <pre> tag generation, without making the message[1] even more complicated? E.g. we could put some zero-width Unicode character at the start.

Thanks as always in advance,

[1] 'flow-topic-participants' => '{{PLURAL:$1|$3 started this topic|{{GENDER:$3|$3}}, {{GENDER:$4|$4}}, {{GENDER:$5|$5}} and $2 {{PLURAL:$2|other|others}}|0=No participation yet|2={{GENDER:$3|$3}} and {{GENDER:$4|$4}}|3={{GENDER:$3|$3}}, {{GENDER:$4|$4}} and {{GENDER:$5|$5}}}}