Use of the Cirrus/Elastic search backend can be enabled on a per-request basis using the useCirrus request parameter. To allow users to test the new search behavior, we want a Gadget that adds this flag to the requests the entity selector widget sends to the wbsearchentities API module.
This can be implemented independently of T162292, but the cirrus based search will not find anything until the Elastic index is rebuild.
Baseline: the gadget allows users to opt in. If they do, they use the cirrus based entity search. This is not visible in the UI.
Opt-in Version: the gadget is always on, but initially only triggers a notification when the search box is focus. The notification gives the user two options: enable cirrus based search, or decline and ignore/hide the notification. The decision is stored in a cookie (this could be changed to user preferences for logged in users).
See also T170549: Provide A/B test for item suggestor for an alternative approach.