Fix useFormAction inheriting ?index param from child route action submission

.changeset/form-action-index.md

@@ -0,0 +1,5 @@
+---
+"react-router-dom": patch
+---
+
+Fix `useFormAction` which was incorrectly inheriting the `?index` query param from child route `action` submissions

packages/react-router-dom/__tests__/data-browser-router-test.tsx

@@ -2693,6 +2693,28 @@ function testDomRouter(
             "/foo"
           );
         });
+
+        it("does not include the index parameter if we've submitted to a child index route", async () => {
+          let router = createTestRouter(
+            createRoutesFromElements(
+              <Route path="/">
+                <Route path="foo">
+                  <Route path="bar" element={<NoActionComponent />}>
+                    <Route index={true} element={<h1>Index</h1>} />
+                  </Route>
+                </Route>
+              </Route>
+            ),
+            {
+              window: getWindow("/foo/bar?index&a=1"),
+            }
+          );
+          let { container } = render(<RouterProvider router={router} />);
+
+          expect(container.querySelector("form")?.getAttribute("action")).toBe(
+            "/foo/bar?a=1"
+          );
+        });
       });
 
       describe("index routes", () => {

packages/react-router-dom/index.tsx

@@ -1551,11 +1551,11 @@ export function useFormAction(
     // would have called useResolvedPath(".") which will never include a search
     path.search = location.search;
 
-    // When grabbing search params from the URL, remove the automatically
-    // inserted ?index param so we match the useResolvedPath search behavior
-    // which would not include ?index
-    if (match.route.index) {
-      let params = new URLSearchParams(path.search);
+    // When grabbing search params from the URL, remove any included ?index param
+    // since it might not apply to our contextual route.  We add it back based
+    // on match.route.index below
+    let params = new URLSearchParams(path.search);
+    if (params.has("index") && params.get("index") === "") {
       params.delete("index");
       path.search = params.toString() ? `?${params.toString()}` : "";
     }