This is a problem with the diff page itself. The diff page overrides the parser output associated with the page and does not pass the toc option.

Something like this in includes/diff/DifferenceEngine.php should fix it:

		if ( $parserOutput ) {
					// Allow extensions to change parser output here
					if ( $this->hookRunner->onDifferenceEngineRenderRevisionAddParserOutput(
						$this, $out, $parserOutput, $wikiPage )
					) {
						$skinOptions = $this->getSkin()->getOptions();
						$out->setSections( $parserOutput->getSections() );
						$out->addParserOutput( $parserOutput, [
							'injectTOC' => $skinOptions['toc'],
							'enableSectionEditLinks' => $this->mNewRevisionRecord->isCurrent()
								&& $this->getAuthority()->probablyCan(
									'edit',
									$this->mNewRevisionRecord->getPage()
								),
							'absoluteURLs' => $this->slotDiffOptions['expand-url'] ?? false
						] );
					}
				}

@JMcLeod_WMF Just want to make sure this in on your radar, thanks!

In T311529#8131057, @Sunpriat2 wrote:

Now it looks like a list with bullets (and numbers)

The skin does not expect it to be within the article so is not loading styles for it. That's what it looks like unstyled.

The fix is described in T311529#8075720

In T311529#8131881, @Jdlrobson wrote:

like unstyled.

Then why were these styles on the first screenshots a month ago?

In T311529#8132255, @Sunpriat2 wrote:

In T311529#8131881, @Jdlrobson wrote:

like unstyled.

Then why were these styles on the first screenshots a month ago?

At that time we were running an A/B test and loading both styles for both variants.

In T311529#8131057, @Sunpriat2 wrote:

Now it looks like a list with bullets (and numbers)

Looks like this is a separate issue, filed as T314254: Templates: CSS classes (toccolours, toc) related to TOC lost styling in Vector-2022.

It doesn't seem that there's something actionable on Parsoid side. Please re-add the tag in case I'm missing something.

@MSantos this is the legacy parser so we need some input from @cscott

The approach proposed in T311529#8075720 seems reasonable to me. We might want to involve @daniel on patch review just because this is "yet another" way that wikitext is turned into a ParserOutput, although in this case at least the ParserOptions aren't involved.

Change 823519 had a related patch set uploaded (by Jdlrobson; author: Jdlrobson):

[mediawiki/core@master] DifferenceEngine: Should respect skin option for rendering table of contents

https://gerrit.wikimedia.org/r/823519

Change 823519 merged by jenkins-bot:

[mediawiki/core@master] DifferenceEngine: Should respect skin option for rendering table of contents

https://gerrit.wikimedia.org/r/823519