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.
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.
On reflection given that the page headings do not render in revisions and the banner is substitution for the heading I actually think this is the correct behaviour. For instance the heading "Download" does not show up in https://www.mediawiki.org/w/index.php?title=Download&type=revision&diff=1896349&oldid=1813316
If we do want to show the banner in the revision diff somewhere someone should raise a separate feature request for that and we should get some design input.