Verbum Comments: prevent duplicate comment submissions

[arcangelini]

Proposed changes

Readers were getting the same comment posted several times — byte-identical content, seconds apart. Verbum's only protection was the submit button's disabled prop, and it leaked in a few ways.

• Guard the comment form's submit event with a re-entrancy check that takes effect synchronously inside the handler, so repeat clicks (or implicit submission from the name/email inputs) can't start a second submission while one is in flight. The button's disabled prop was previously applied a macrotask late via setTimeout( …, 0 ); three clicks in a single task all reached the server.
• Stop re-posting the form once the request has already reached wp-comments-post.php. The subscription-modal path called HTMLFormElement.prototype.submit() on any non-redirect response — and re-enabled the button first — so one click could produce two submissions. The error branch had no return, so a rejected comment showed the error inline and sent a second POST.
• Report failures instead of hanging: an aborted fetch used to leave the button stuck in its busy state with no message and no way to retry.
• Release the guard on a back/forward-cache restore (pageshow + persisted), so navigating back to a post doesn't leave the comment form permanently locked.

Worth noting for reviewers: WordPress's own duplicate-comment check in wp_allow_comment() does not reliably reject these, so the client is currently the only thing standing between a reader and a duplicate.

Related product discussion/links

• CM-847

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

No.

Testing instructions

Verbum only runs on WordPress.com Simple, so test on a Simple site with comments open to everyone.

• Open a post as a logged-out reader, fill in name and email, and write a comment.
• Click Comment several times in quick succession while the request is still in flight. Exactly one comment should be created — before this change each extra click could add another.
• Confirm the happy path is unchanged: the comment posts and you land back on the post, with the subscribe modal first if it applies to your account.
• Make the server reject a submission (for example clear the required name or email before submitting). You should get the error message inline, the comment text should stay in the box, and the button should become clickable again — with no second POST and no full-page navigation to the raw error page.
• After commenting, hit the browser Back button and confirm the comment form is usable again rather than stuck disabled.

[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/verbum-duplicate-comment-submissions branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/verbum-duplicate-comment-submissions

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!

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/src/index.tsx	0/290 (0.00%)	0.00%	37 💔
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/class-verbum-comments.php	0/379 (0.00%)	0.00%	1 ❤️‍🩹

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.}