Forms: conditional logic (4/5) — server-side render and validation

[enejb]

Part 4 of 5. This series splits #50938 into reviewable pieces. Each PR is based on the previous one, so the diff shown here is incremental.

1. Forms: conditional logic (1/5) — vocabulary and feature flag #50976 — vocabulary and feature flag
2. Forms: conditional logic (2/5) — the resolver, in JS and PHP #50977 — the resolver, in JS and PHP
3. Forms: conditional logic (3/5) — apply in the browser #50978 — apply in the browser
4. Forms: conditional logic (4/5) — server-side render and validation #50979 — server-side render and validation
5. Forms: conditional logic (5/5) — the editor panel #50980 — the editor panel

Proposed changes

Part 4 of 5 splitting #50938. Two problems the browser cannot solve on its own.

A required field that conditional logic hides used to block submission. It was invalid, but the visitor could not see it to fill it in. Validation now skips fields the rules hide, evaluated server-side from the submitted values rather than trusting anything the browser reports. Parse-time validation is deferred for conditional fields, which otherwise wrote a static error before any answers existed.

Fields that start hidden used to render visible and then disappear once the runtime booted. Initial visibility is now resolved while rendering and stamped onto the markup, so the form is correct in its first paint. Values prefilled through the query string are taken into account, so a link that prefills the field a rule depends on renders the resulting state directly.

Only hidden fields are dropped from the stored feedback, so a response records what the visitor was actually asked.

Related product discussion/links

• https://wp.me/p1HpG7-xBk

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

A form response no longer stores values for fields that conditional logic hid at submit time. Nothing new is collected.

Testing instructions

With the flag on and a form that hides a required field:

• Submit without revealing the hidden required field → submission succeeds, no error about a field you cannot see.
• Reveal it, leave it empty, submit → validation fails as normal.
• Load a form whose first field is hidden by a rule → it must not flash visible before disappearing. Throttle the network to make a flash obvious.
• Load the same form with ?fieldname=value prefilling the field a rule depends on → the dependent field renders in its resulting state immediately.
• Check the stored response contains only fields that were visible.

Also: jetpack test php packages/forms.

[github-actions]

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Code Coverage Summary

No summary data is available for parent commit 5a56544, so cannot calculate coverage changes. 😴

If that commit is a feature branch rather than a trunk commit, this is expected. Otherwise, this should be updated once coverage for 5a56544 is available.

Full summary · PHP report · JS report