(cross-posting to ops@ as requested. This is in regards to an EventLogging schema[1] applied in javascript to track user behavior on pages they find via internal search to measure the quality of the search results[2][3]. )
The referrer would be nice to use, but we are trying to track more than just a single click away from Special:Search. We are trying to track all pages the user visits (for 10 minutes) by clicking from search results to one page and then another. The other difficulty here is we are trying to track only the pages the user clicked to coming from our search, and not a page they found by repeating their search on google and clicking through to Wikipedia.
Using the wprov[4] query parameter to declare the provenance as desktop search should avoid all the issues mentioned above. Varnish strips wprov so the varnish cache remains unfragmented. The javascript can pickup on the parameter and log the necessary events.