Verbum Comments: fix blocked commenting from cookies

[arcangelini]

Fixes CM-841

Proposed changes

When a site requires visitors to log in before commenting, Verbum can end up in a dead end: the browser blocks the cookies the login buttons depend on, so no login options render, but the email form is still offered. The visitor writes a comment, submits it, and gets "Sorry, you must be logged in to comment."

• Hide the email form when login is required — it can't produce an accepted comment.
• Explain the situation instead: a short notice saying commenting needs cookie access here.
• Disable the Comment button, and ignore the submit event if the form is submitted another way.

Only affects visitors who are already blocked. If cookies are readable, nothing changes.

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

No.

Testing instructions

You need an Atomic site with Settings → Discussion → "Users must be registered and logged in to comment" enabled, plus a browser that blocks third-party cookies (Safari by default, or Chrome with third-party cookies blocked).

Atomic matters: Verbum runs inside the Jetpack comments iframe there, so it's third-party content and its cookies get blocked. On Simple the form is first-party and this path never triggers.

• Open a post on that site as a logged-out visitor.
• Start typing in the comment box so the panel below it opens.
• Before: name/email fields appear. Filling them in and submitting gets you "Sorry, you must be logged in to comment."
• After: no email form. You get a notice reading "Your browser is blocking the cookies needed to log in and comment here. Allow cookies in your privacy settings, then reload the page." and the Comment button stays disabled.
• Allow cookies for the site, reload, and confirm the WordPress.com and Facebook login buttons come back and commenting works as before.

Also worth a pass on a site without the login requirement, in the same browser, to confirm the email form still shows and guest commenting is untouched.

[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 cm-841/surface-login-requirement branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin cm-841/surface-login-requirement

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]

Thank you for your PR!

When contributing to Jetpack, we have a few suggestions that can help us test and review your patch:

• ✅ Include a description of your PR changes.
  
• ✅ Add a "[Status]" label (In Progress, Needs Review, ...).
  
• ✅ Add testing instructions.
  
• ✅ Specify whether this PR includes any changes to data or privacy.
  
• ✅ Add changelog entries to affected projects
  

This comment will be updated as you work on your PR and make changes. If you think that some of those checks are not needed for your PR, please explain why you think so. Thanks for cooperation 🤖

---

Follow this PR Review Process:

1. Ensure all required checks appearing at the bottom of this PR are passing.
2. Make sure to test your changes on all platforms that it applies to. You're responsible for the quality of the code you ship.
3. You can use GitHub's Reviewers functionality to request a review.
4. When it's reviewed and merged, you will be pinged in Slack to deploy the changes to WordPress.com simple once the build is done.

If you have questions about anything, reach out in #jetpack-developers for guidance!

[jp-launch-control]

Code Coverage Summary

Coverage changed in 4 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/src/state.tsx	0/156 (0.00%)	0.00%	17 💔
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/src/components/logged-out.tsx	0/165 (0.00%)	0.00%	10 💔
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/class-verbum-comments.php	0/378 (0.00%)	0.00%	1 ❤️‍🩹
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/src/index.tsx	0/253 (0.00%)	0.00%	-3 💚

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}