fix: client prerender fallback (#10295)

* fix: client prerender fallback

* Use Speculation Rules prefetch as prerender fallback instead of link tag, improve changeset

* Update .changeset/warm-spoons-fetch.md

---------

Co-authored-by: Bjorn Lu <bjornlu.dev@gmail.com>

.changeset/warm-spoons-fetch.md

@@ -0,0 +1,5 @@
+---
+"astro": patch
+---
+
+Adds a prefetch fallback when using the `experimental.clientPrerender` option. If prerendering fails, which can happen if [Chrome extensions block prerendering](https://developer.chrome.com/blog/speculation-rules-improvements#chrome-limits), it will fallback to prefetching the URL. This works by adding a `prefetch` field to the `speculationrules` script, but does not create an extra request.

packages/astro/src/@types/astro.ts

@@ -1653,7 +1653,7 @@ export interface AstroUserConfig {
 		 * @description
 		 * Enables pre-rendering your prefetched pages on the client in supported browsers.
 		 *
-		 * This feature uses the experimental [Speculation Rules Web API](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API) and overrides the default `prefetch` behavior globally to prerender links on the client.
+		 * This feature uses the experimental [Speculation Rules Web API](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API) and enhances the default `prefetch` behavior globally to prerender links on the client.
 		 * You may wish to review the [possible risks when prerendering on the client](https://developer.mozilla.org/en-US/docs/Web/API/Speculation_Rules_API#unsafe_prefetching) before enabling this feature.
 		 *
 		 * Enable client side prerendering in your `astro.config.mjs` along with any desired `prefetch` configuration options:

packages/astro/src/prefetch/index.ts

@@ -330,6 +330,15 @@ function appendSpeculationRules(url: string) {
 				urls: [url],
 			},
 		],
+		// Currently, adding `prefetch` is required to fallback if `prerender` fails.
+		// Possibly will be automatic in the future, in which case it can be removed.
+		// https://github.com/WICG/nav-speculation/issues/162#issuecomment-1977818473
+		prefetch: [
+			{
+				source: 'list',
+				urls: [url],
+			},
+		],
 	});
 	document.head.append(script);
 }