Summary

• Fixes Partial "messages" not found. Skipping... #3473 — "Partial 'messages' not found. Skipping..."
• Forms inside f-client-nav without an explicit f-partial attribute were being intercepted as partial requests, breaking normal form submissions

Root cause

The form submit handler's fallback chain for determining the partial URL:

const rawPartialUrl = e.submitter?.getAttribute(PARTIAL_ATTR) ??
  e.submitter?.getAttribute("formaction") ??
  el.getAttribute(PARTIAL_ATTR) ?? el.action;

Always falls through to el.action, which defaults to the current page URL for any <form> without an explicit action attribute. Since el.action is never empty, the rawPartialUrl !== "" check on the next line was always true, meaning every form inside f-client-nav was intercepted — even forms that never opted in to partial navigation.

This caused:

1. Normal form POSTs to be sent with ?fresh-partial=true
2. Server rendering in partial mode, returning only <Partial> fragments
3. The rest of the page content being lost

Fix

Added a hasExplicitPartial check that requires the form or its submitter to have an explicit f-partial or formaction attribute before intercepting. Forms without these attributes now submit normally, even inside f-client-nav.

Test plan

• Lint/format clean
• Manual test: form inside f-client-nav without f-partial should submit normally (full page reload)
• Manual test: form with f-partial inside f-client-nav should still work as partial navigation

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