fix: add warning for form action responses lost without SSR

.changeset/many-cats-breathe.md

@@ -0,0 +1,5 @@
+---
+"@sveltejs/kit": patch
+---
+
+fix: warn when form action responses are lost because SSR is off

packages/kit/src/runtime/server/page/index.js

@@ -16,6 +16,7 @@ import { respond_with_error } from './respond_with_error.js';
 import { get_option } from '../../../utils/options.js';
 import { get_data_json } from '../data/index.js';
 import { load_page_nodes } from './load_page_nodes.js';
+import { DEV } from 'esm-env';
 
 /**
  * The maximum request depth permitted before assuming we're stuck in an infinite loop
@@ -99,6 +100,28 @@ export async function render_page(event, page, options, manifest, state, resolve
 		// no server data to prerender. As a result, the load functions and rendering
 		// only occur client-side.
 		if (get_option(nodes, 'ssr') === false && !(state.prerendering && should_prerender_data)) {
+			// if SSR is turned off, and the user makes a request through a non-enhanced form,
+			// the returned value is lost because neither the server nor the client will handle it
+			if (DEV) {
+				const no_enhance = !event.request.headers.has('x-sveltekit-action');
+
+				if (action_result && no_enhance) {
+					if (action_result.type === 'error') {
+						console.warn(
+							"The form action returned an error, but +error.svelte wasn't rendered because SSR is off. To get the error page with CSR, enhance your form with `use:enhance`. See https://kit.svelte.dev/docs/form-actions#progressive-enhancement-use-enhance"
+						);
+					}
+					// for type: 'success' and type: 'failure'
+					else if (action_result.data) {
+						/// case: lost data
+						console.warn(
+							"The form action returned a value, but it isn't available in `$page.form`, because SSR is off. To handle the returned value in CSR, enhance your form with `use:enhance`. See https://kit.svelte.dev/docs/form-actions#progressive-enhancement-use-enhance"
+						);
+					}
+					// type 'redirect' is returned immediately, so we don't need to handle it here
+				}
+			}
+
 			return await render_response({
 				branch: [],
 				fetched,