fix(server-functions): only throw SSR guard when 'window' is undefined

Previously the SSR‐guard in `server-functions.ts` unconditionally threw
whenever `isServer` was true,
which blocked invocation of `routeAction$` under Vitest+JSDOM (and the
QwikCityMockProvider) in user tests.

Now we narrow the guard to:

if (isServer && typeof window === 'undefined') {
  throw …;
}

After this fix:

- True SSR(no 'window') still throws an error as beforev
- JSDOM/Vitest tests(and browsers) have a global 'window', do not throw
the error, and returns a promise

This will unblock testing of actions in JSDOM environments without
impacting real SSR safety

Closes QwikDev#5874

Author: LogProphet

.changeset/lovely-cycles-pay.md

@@ -0,0 +1,22 @@
+---
+'@builder.io/qwik-city': patch
+---
+
+FIX: Bug #5874 - SSR guard: only throw in true SSR (when window is undefined), allowing actions to run in Vitest tests
+
+How this change works:
+
+Previously the SSR‐guard in `server-functions.ts` unconditionally threw whenever `isServer` was true,  
+which blocked invocation of `routeAction$` under Vitest+JSDOM (and the QwikCityMockProvider) in user tests.  
+
+Now we narrow the guard to:
+if (isServer && typeof window === 'undefined') {
+  throw …;
+}
+
+This will ensure:
+
+- True SSR (no window) still throws as before.
+- JSDOM/Vitest tests (and browsers) have a global window, skip the throw, and return a Promise.
+
+This change unblocks testing of actions in JSDOM environments without impacting real SSR safety.

packages/qwik-city/src/runtime/src/server-functions.ts

@@ -88,7 +88,8 @@ export const routeActionQrl = ((
     });
 
     const submit = $((input: unknown | FormData | SubmitEvent = {}) => {
-      if (isServer) {
+      // only throw in true SSR (no mock context)
+      if (isServer && typeof window === 'undefined') {
         throw new Error(`Actions can not be invoked within the server during SSR.
 Action.run() can only be called on the browser, for example when a user clicks a button, or submits a form.`);
       }