Is there a way to detect that we are viewing a diff page, other than resorting to ArticleContentOnDiff hook.

You could check for the existence of the diff parameter that should be enough.

		$diff = $request->getVal( 'diff' );

Change 235492 had a related patch set uploaded (by Sumit):
WikidataPageBanner disable on diff pages

https://gerrit.wikimedia.org/r/235492

Is that a good idea? Patrolling changes to the pagebanner parameters seems kind of annoying without seeing the effect. The banner should be on top of the article, not top of the page, but otherwise it seems useful to have it.

Change 235492 merged by jenkins-bot:
WikidataPageBanner disable on diff pages

https://gerrit.wikimedia.org/r/235492

@Nicolas_Raoul thoughts on @Tgr's comment [1]? Should banners be showing underneath the diff itself?
@Sumit any idea why they were showing up where they were?

[1] https://phabricator.wikimedia.org/T110384#1598347

From a user experience point of view, I agree with Tgr that the banner should appear between the diff (or source edit box) and the actual page content, so that the preview is as close to reality as possible.

Banner has been showing above diff because by nature it is meant to be the first element of page body. For this it uses OutputPage::prependHtml() on page body. In case of a diff, it does not distinguish it from other content.

Mmm... is there any way we can add it when encountered rather than use prependHtml?
I guess this will cause problems with multiple uses of the banner code or adding a banner at the bottom of the page causing it to render at the bottom?

How does the diff get added?
It's worth noticing the mobile diffs do not give a preview at all.

Using the mechanism in https://gerrit.wikimedia.org/r/#/c/238654/ we should be able to inject it under the revisions... in theory... not sure how problematic not having this is to the community in the meantime.