Post Stream Search technically adds a new API search endpoint for posts that could be used to search across all discussions, but it only adds a UI that uses that endpoint with a discussion ID filter.
A basic site-wide post search should actually be pretty easy to implement. The searcher class is simple and can be copied directly from Post Stream Search. The UI doesn't exist, but it just needs to render a discussion title, the post body and a link.
The biggest challenge with post search is probably best-match sorting. Post Stream Search doesn't need to sort results because they stay in the original discussion order. Flarum's built-in discussion search is already not always the best, translating this to posts where there will be even more results per keyword will probably lead to suboptimal results.
With or without best-match sorting, there will probably be an issue with too many posts being matched by simple keywords. Some sort of grouping or additional filters seem almost required. Like "search only first post", "only show x posts per discussion" (technically, that's what Flarum's discussion search does, it picks the best matching post per discussion)
If you make post search your default search, there is then the challenge of discussion titles. You can't just ignore them, but you cannot return every post from the matching discussions either.
Maybe a compromise would be to perform the search & results display exactly like the current built-in discussion search, but add an information/option next/underneath each result to say how many posts in the discussion seem to match the query, and offer to expand the results to show more than 1 post. I'm not really sure how to implement this in an efficient way using MySQL text search though.
