fix: clear stale wpcom_user_id in logged-out Write editor to prevent anon post loss

[allilevine]

Fixes READ-574

Proposed changes

• The anonymous (logged-out) Write editor and the Calypso /setup/write-on signup flow share the wordpress.com origin and its localStorage. Calypso bootstraps whether a visitor is logged in from the wpcom_user_id key, and the write-on flow blocks entry and redirects to My Home when it believes the visitor is already authenticated — so it never transfers the draft that Publish hands off.
• When a prior session's logout fails to clear wpcom_user_id, a genuinely logged-out visitor is treated as authenticated and their anonymous post is silently dropped.
• This PR clears the stale wpcom_user_id when the logged-out editor loads, so the persisted auth state matches the real (logged-out) session before the Publish handoff. The clear is scoped to that single key on purpose — clearing the whole store would also wipe the wpcom-write-anon-draft snapshot the editor just persisted. It is gated on the existing isAnon() flag, so the logged-in editor is unaffected.

Related product discussion/links

• READ-574

Does this pull request change what data or activity we track or use?

Yes — adds one Tracks event, wpcom_write_editor_anon_stale_user_cleared, fired only when a stale wpcom_user_id was actually found and removed on the anonymous editor. No new personal data is collected.

Testing instructions

Because the full reproduction spans the wpcom (Atlas) anonymous route and the Calypso /setup/write-on flow, the end-to-end path is exercised on wordpress.com rather than a vanilla local WordPress. The editor-side behavior was verified locally by loading the served view.js in a real browser with the anon flag set and a seeded localStorage:

• With window.wpcomWriteIsAnon = true and a pre-seeded wpcom_user_id, the key is removed on load, the wpcom-write-anon-draft snapshot is preserved, and the wpcom_write_editor_anon_stale_user_cleared Tracks event fires.
• With the anon flag off (logged-in editor), wpcom_user_id is left untouched and no event fires.

Full-flow check on wordpress.com:

• While logged out, write a post in Write and click Publish; create a new account.
• Confirm you land back in the Write editor with the post intact (not on My Home).
• Log out, then repeat — even when a wpcom_user_id lingered in localStorage from the previous run, the post should survive signup.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (WordPress.com Site Helper), and enable the fix/write-anon-clear-stale-user-id branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/write-anon-clear-stale-user-id

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/write/view.js	0/6422 (0.00%)	0.00%	39 💔

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}