fix: don't intercept forms without explicit f-partial inside f-client-nav (#3722)

Fixes #3473

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

packages/fresh/src/runtime/client/partials.ts

@@ -231,12 +231,21 @@ document.addEventListener("submit", async (e) => {
       return;
     }
 
+    const hasExplicitPartial = e.submitter?.hasAttribute(PARTIAL_ATTR) ||
+      e.submitter?.hasAttribute("formaction") ||
+      el.hasAttribute(PARTIAL_ATTR) ||
+      el.hasAttribute("action");
+
     const rawPartialUrl = e.submitter?.getAttribute(PARTIAL_ATTR) ??
       e.submitter?.getAttribute("formaction") ??
       el.getAttribute(PARTIAL_ATTR) ?? el.action;
     const rawActionUrl = e.submitter?.getAttribute("formaction") ?? el.action;
 
-    if (rawPartialUrl !== "") {
+    // Only intercept forms that explicitly opt in to partial navigation
+    // via f-partial, formaction, or an explicit action attribute. Without
+    // this check, every form inside f-client-nav would be intercepted
+    // because el.action is always non-empty (defaults to the current URL).
+    if (hasExplicitPartial && rawPartialUrl !== "") {
       e.preventDefault();
 
       const partialUrl = new URL(rawPartialUrl, location.href);

packages/fresh/tests/partials_test.tsx

@@ -1858,6 +1858,49 @@ Deno.test({
   },
 });
 
+Deno.test({
+  name: "partials - form without action inside f-client-nav not intercepted",
+  fn: async () => {
+    const app = testApp()
+      .post("/", (ctx) => {
+        return ctx.render(
+          <Doc>
+            <p class="submitted">form submitted normally</p>
+          </Doc>,
+        );
+      })
+      .get("/", (ctx) => {
+        return ctx.render(
+          <Doc>
+            <div f-client-nav>
+              <form method="post">
+                <input name="name" value="foo" />
+                <Partial name="foo">
+                  <p class="init">init</p>
+                </Partial>
+                <button type="submit" class="update">
+                  update
+                </button>
+              </form>
+            </div>
+          </Doc>,
+        );
+      });
+
+    await withBrowserApp(app, async (page, address) => {
+      await page.goto(address, { waitUntil: "load" });
+      await page.locator(".init").wait();
+
+      // Form should do a full page navigation, not a partial update
+      await Promise.all([
+        page.waitForNavigation(),
+        page.locator(".update").click(),
+      ]);
+      await page.locator(".submitted").wait();
+    });
+  },
+});
+
 Deno.test({
   name: "partials - submit form redirect",
   fn: async () => {