With Zotero 10, this is possible but I have to multi-select all the libraries one at a time before performing the search. Is there an easier way to do this?
No, that's just a misunderstanding. The quick search is a filter — it filters whatever you're currently looking at. That could be a library or multiple libraries, or it could just be a collection, the trash, whatever. It's also affected by whether you have tags selected in the tag selector.
Those quick search modes — "Title, Creator, Year", "All Fields & Tags", and "Everything" — are describing the content it's filtering by on the visible items and have nothing to do with the scope of the search.
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Still, it would be nice if this was actually true without doing that extra step, so I don't fall into the assumption that it is, in fact, not "everything"
https://s3.amazonaws.com/zotero.org/images/forums/u7359284/nvj7rpq2lurwd2j10gim.png
Those quick search modes — "Title, Creator, Year", "All Fields & Tags", and "Everything" — are describing the content it's filtering by on the visible items and have nothing to do with the scope of the search.