fix: don't automatically prerender non-SSR'd pages (#9352)

reverts #8131
This reverts the optimisation wherein pages with ssr = false and no explicit prerender = false would automatically be prerendered, as it turns out to be unsafe.

fixes #9275
fixes #8428

.changeset/cuddly-kangaroos-rush.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't automatically prerender non-SSR'd pages

packages/kit/src/core/postbuild/analyse.js

@@ -122,15 +122,7 @@ async function analyse({ manifest_path, env }) {
 				validate_common_exports(page.universal, page.universal_id);
 			}
 
-			const should_prerender = get_option(nodes, 'prerender');
-			prerender =
-				should_prerender === true ||
-				// Try prerendering if ssr is false and no server needed. Set it to 'auto' so that
-				// the route is not removed from the manifest, there could be a server load function.
-				// People can opt out of this behavior by explicitly setting prerender to false
-				(should_prerender !== false && get_option(nodes, 'ssr') === false && !page?.server?.actions
-					? 'auto'
-					: should_prerender ?? false);
+			prerender = get_option(nodes, 'prerender') ?? false;
 
 			config = get_config(nodes);
 		}

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

@@ -79,38 +79,13 @@ export async function render_page(event, page, options, manifest, state, resolve
 		// it's crucial that we do this before returning the non-SSR response, otherwise
 		// SvelteKit will erroneously believe that the path has been prerendered,
 		// causing functions to be omitted from the manifesst generated later
-		const should_prerender = get_option(nodes, 'prerender');
-
+		const should_prerender = get_option(nodes, 'prerender') ?? false;
 		if (should_prerender) {
 			const mod = leaf_node.server;
 			if (mod?.actions) {
 				throw new Error('Cannot prerender pages with actions');
 			}
 		} else if (state.prerendering) {
-			// Try to render the shell when ssr is false and prerendering not explicitly disabled.
-			// People can opt out of this behavior by explicitly setting prerender to false.
-			if (
-				should_prerender !== false &&
-				get_option(nodes, 'ssr') === false &&
-				!leaf_node.server?.actions
-			) {
-				return await render_response({
-					branch: [],
-					fetched: [],
-					page_config: {
-						ssr: false,
-						csr: get_option(nodes, 'csr') ?? true
-					},
-					status,
-					error: null,
-					event,
-					options,
-					manifest,
-					state,
-					resolve_opts
-				});
-			}
-
 			// if the page isn't marked as prerenderable, then bail out at this point
 			return new Response(undefined, {
 				status: 204