Fix "sticky" GA4 search tracker tool_name

The GA4 search tracker was only setting the `tool_name` on its `search`
event in case the user had accepted an autocomplete suggestion. Now that
we persist the `tool_name` in the page's analytics state, this meant
that if the user had accepted a suggestion on a previous search, the
`tool_name` would continue to be set for subsequent searches regardless
of whether they've actually _used_ autocomplete.

CHANGELOG.md

@@ -7,6 +7,10 @@
   useful summary for people upgrading their application, not a replication
   of the commit log.
 
+## Unreleased
+
+* Fix "sticky" GA4 search tracker `tool_name` ([PR #4422](https://github.com/alphagov/govuk_publishing_components/pull/4422))
+
 ## 45.6.0
 
 * Fix yellow focus colour overspill ([PR #4418](https://github.com/alphagov/govuk_publishing_components/pull/4418))

app/assets/javascripts/govuk_publishing_components/analytics-ga4/ga4-search-tracker.js

@@ -65,6 +65,11 @@ window.GOVUK.Modules = window.GOVUK.Modules || {};
           // part of the `page_view` event on the subsequent page load
           window.sessionStorage &&
             window.sessionStorage.setItem('searchAutocompleteAccepted', 'true')
+        } else {
+          // Explicitly set `tool_name` to `null` if the user has not accepted a suggestion, as
+          // `undefined` will not overwrite the current value in the analytics state (meaning a
+          // previously set value would be used if there was one)
+          data.tool_name = null
         }
 
         data.length = Number(this.$searchInput.dataset.autocompleteSuggestionsCount)

spec/javascripts/govuk_publishing_components/analytics-ga4/ga4-search-tracker.spec.js

@@ -145,15 +145,15 @@ describe('Google Analytics search tracking', () => {
       expect(setItemSpy).toHaveBeenCalledWith('searchAutocompleteAccepted', 'true')
     })
 
-    it('does not set tool_name if the user has not accepted a suggestion', () => {
+    it('sets tool_name to null if the user has not accepted a suggestion', () => {
       input.dataset.autocompleteTriggerInput = 'i want to'
       input.dataset.autocompleteSuggestions = 'i want to fish|i want to dance|i want to sleep'
       input.dataset.autocompleteSuggestionsCount = '3'
 
       input.value = 'i want to fish'
       GOVUK.triggerEvent(form, 'submit')
 
-      expect(sendSpy.calls.mostRecent().args[0].tool_name).toBeUndefined()
+      expect(sendSpy.calls.mostRecent().args[0].tool_name).toBeNull()
     })
   })