The wikipage.content JavaScript hook exists to allow gadgets and scripts to react to HTML content dynamically changing in the page. The visual diff feature adds HTML content but does not fire this hook (or, as far as I can tell, any other hook) which makes it impossible for user scripts and gadgets to add expected features to that rendered content.
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Comment Actions
It seems to fire for me -- if I look at https://en.wikipedia.org/w/index.php?title=Battle_of_the_Saw&diff=prev&oldid=1175331259 then mw.hook('wikipage.content').add((content) => console.log('wikipage.content', content)); shows an appropriate element. It doesn't trigger when you toggle between visual and wikitext, but that doesn't actually change the content so it probably shouldn't.
Comment Actions
Are you seeing it trigger for the visual diff, or only for the preview under the diff? Because I only see it trigger for the latter.
Comment Actions
It triggers on the div#mw-content-text.mw-body-content that contains the diff and the preview under it. Though probably not with helpful timing -- I imagine that at the moment of it triggering the diff content isn't inserted yet.
That said, I don't know that the content in the visual diff should be bundled into wikipage.content -- it's a strange and heavily processed snippet of the page, and anything that meddles with it without knowing exactly what it's doing using the same behavior as it applies to real wiki content is likely to mess up the diff.
Comment Actions
Well, right now it fires no hook at all, so scripts have little hope of dealing with it in a reliable manner.