Write On: thread source through the anon flow (#113395)

* Write On: thread `source` query param through the anon flow

Read `source` from the query string (mirroring write-new-site) and carry it
into the `calypso_write_on_flow_entered` Tracks event and the Write editor
redirect, so the anon funnel's source survives signup for attribution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D3rtpQqCkgRJvNLFpFd4Qn

* Write On: add positive test for source forwarding into the redirect

Mirror write-new-site's forwarding test — set `source` in the query string
and assert it lands in the Write editor redirect URL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D3rtpQqCkgRJvNLFpFd4Qn

* Write On: sanitize and omit-when-empty the source param

Normalize `source` to [a-z0-9_-] lowercased and omit the Tracks prop when
empty, matching the anon funnel's events (wpcom/jetpack) so the whole journey
reports one consistent value. Add a sanitization test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D3rtpQqCkgRJvNLFpFd4Qn

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: allilevine

client/landing/stepper/declarative-flow/flows/write-on/test/write-on.test.ts

@@ -9,6 +9,8 @@ import writeOn, { ANON_DRAFT_STORAGE_KEY, MAX_DRAFT_SIZE } from '../write-on';
 
 const mockIsEnabled = jest.fn( ( flag: string ) => flag === 'calypso/write-on-flow' );
 
+let mockSearch = '';
+
 jest.mock( '@automattic/calypso-config', () => {
 	const fn = Object.assign( ( key: string ) => key, {
 		isEnabled: ( flag: string ) => mockIsEnabled( flag ),
@@ -37,6 +39,10 @@ jest.mock( 'calypso/landing/stepper/stores', () => ( {
 	ONBOARD_STORE: 'ONBOARD_STORE',
 } ) );
 
+jest.mock( '../../../../hooks/use-query', () => ( {
+	useQuery: () => new URLSearchParams( mockSearch ),
+} ) );
+
 jest.mock( 'calypso/landing/stepper/utils/steps-with-required-login', () => ( {
 	stepsWithRequiredLogin: ( steps: unknown ) => steps,
 } ) );
@@ -62,6 +68,7 @@ describe( 'write-on flow', () => {
 
 	beforeEach( () => {
 		jest.clearAllMocks();
+		mockSearch = '';
 		mockIsEnabled.mockImplementation( ( flag: string ) => flag === 'calypso/write-on-flow' );
 		window.localStorage.clear();
 		Object.defineProperty( window, 'location', {
@@ -136,6 +143,44 @@ describe( 'write-on flow', () => {
 		} );
 	} );
 
+	it( 'forwards the source query param into the Write editor redirect', async () => {
+		mockSearch = 'source=post_new';
+		window.localStorage.setItem(
+			ANON_DRAFT_STORAGE_KEY,
+			JSON.stringify( { title: 'My title', content: '<p>Body</p>', ts: 1 } )
+		);
+		wpcomPostMock.mockResolvedValue( { ID: 42 } );
+
+		await submitFor( 'processing', {
+			processingResult: ProcessingResult.SUCCESS,
+			siteId: 99,
+			siteSlug: 'example.wordpress.com',
+		} );
+
+		expect( window.location.assign ).toHaveBeenCalledWith(
+			'https://example.wordpress.com/wp-admin/admin.php?page=write&post=42&source=post_new'
+		);
+	} );
+
+	it( 'sanitizes the source query param to match the anon funnel before forwarding', async () => {
+		mockSearch = 'source=Post_New%21';
+		window.localStorage.setItem(
+			ANON_DRAFT_STORAGE_KEY,
+			JSON.stringify( { title: 'My title', content: '<p>Body</p>', ts: 1 } )
+		);
+		wpcomPostMock.mockResolvedValue( { ID: 42 } );
+
+		await submitFor( 'processing', {
+			processingResult: ProcessingResult.SUCCESS,
+			siteId: 99,
+			siteSlug: 'example.wordpress.com',
+		} );
+
+		expect( window.location.assign ).toHaveBeenCalledWith(
+			'https://example.wordpress.com/wp-admin/admin.php?page=write&post=42&source=post_new'
+		);
+	} );
+
 	it( 'preserves the localStorage draft, logs to logstash, and lands on the site home when the POST fails', async () => {
 		window.localStorage.setItem(
 			ANON_DRAFT_STORAGE_KEY,

client/landing/stepper/declarative-flow/flows/write-on/write-on.ts

@@ -8,6 +8,7 @@ import { logToLogstash } from 'calypso/lib/logstash';
 import wpcom from 'calypso/lib/wp';
 import { useSelector } from 'calypso/state';
 import { isUserLoggedIn } from 'calypso/state/current-user/selectors';
+import { useQuery } from '../../../hooks/use-query';
 import { ONBOARD_STORE } from '../../../stores';
 import { stepsWithRequiredLogin } from '../../../utils/steps-with-required-login';
 import { STEPS } from '../../internals/steps';
@@ -58,6 +59,14 @@ function clearAnonDraft() {
 	}
 }
 
+// Normalize the funnel `source` to [a-z0-9_-], lowercased first — kept identical
+// to the anon funnel's reader (wpcom's write-editor-anon.php / jetpack's view.js,
+// both mirroring PHP sanitize_key) so the whole journey reports one consistent
+// value. Empty string when absent, so callers can omit the prop entirely.
+function sanitizeSource( raw: string | null ): string {
+	return ( raw || '' ).toLowerCase().replace( /[^a-z0-9_-]/g, '' );
+}
+
 function initialize() {
 	// The flag is a kill switch for the flow. When it is off, redirect to the
 	// standard onboarding flow so there is nothing to land users on.
@@ -79,6 +88,11 @@ const writeOn: FlowV2< typeof initialize > = {
 		const isLoggedIn = useSelector( isUserLoggedIn );
 		const hasRunEntryChecks = useRef( false );
 
+		// The anon entry point links here with a `source` (e.g. `?source=post_new`).
+		// Stepper preserves the query string across steps, so it is still present at
+		// entry — carry it into the funnel's Tracks events for attribution.
+		const source = sanitizeSource( useQuery().get( 'source' ) );
+
 		// Entry checks must fire at most once per mount. Re-running them on a
 		// later isLoggedIn flip (e.g. mid-signup) would redirect the user out of
 		// the flow during the auth round-trip.
@@ -110,14 +124,19 @@ const writeOn: FlowV2< typeof initialize > = {
 
 			recordTracksEvent( 'calypso_write_on_flow_entered', {
 				draft_size: draft.title.length + draft.content.length,
+				...( source ? { source } : {} ),
 			} );
 
 			if ( draft.title ) {
 				setSiteTitle( draft.title );
 			}
-		}, [ currentStepSlug, isLoggedIn, setSiteTitle ] );
+		}, [ currentStepSlug, isLoggedIn, setSiteTitle, source ] );
 	},
 	useStepNavigation( currentStepSlug, navigate ) {
+		// Same `source` the entry point carried in — forward it into the Write
+		// editor so its back button and Tracks stay attributed to the funnel.
+		const source = sanitizeSource( useQuery().get( 'source' ) );
+
 		const submit: SubmitHandler< typeof initialize > = async ( submittedStep ) => {
 			const { slug, providedDependencies } = submittedStep;
 			switch ( slug ) {
@@ -153,8 +172,12 @@ const writeOn: FlowV2< typeof initialize > = {
 							site_id: siteId,
 						} );
 
+						const params = new URLSearchParams( { page: 'write', post: String( post.ID ) } );
+						if ( source ) {
+							params.set( 'source', source );
+						}
 						window.location.assign(
-							`https://${ siteSlug }/wp-admin/admin.php?page=write&post=${ post.ID }`
+							`https://${ siteSlug }/wp-admin/admin.php?${ params.toString() }`
 						);
 					} catch ( error ) {
 						// Surfacing the failure to the user is handled upstream by