docs: document the behaviour of paths on fallback pages (#11267)

addresses #10235

documentation/docs/25-build-and-deploy/55-single-page-apps.md

@@ -35,6 +35,8 @@ The `fallback` page is an HTML page created by SvelteKit from your page template
 
 On some hosts it may be `index.html` or something else entirely — consult your platform's documentation.
 
+> Note that the fallback page will always contain absolute asset paths (i.e. beginning with `/` rather than `.`) regardless of the value of [`paths.relative`](/docs/configuration#paths), since it is used to respond to requests for arbitrary paths.
+
 ## Apache
 
 To run an SPA on [Apache](https://httpd.apache.org/), you should add a `static/.htaccess` file to route requests to the fallback page:
@@ -58,4 +60,4 @@ If you want certain pages to be prerendered, you can re-enable `ssr` alongside `
 /// file: src/routes/my-prerendered-page/+page.js
 export const prerender = true;
 export const ssr = true;
-```
+```

packages/kit/src/exports/public.d.ts

@@ -478,6 +478,8 @@ export interface KitConfig {
 		 * If `true`, `base` and `assets` imported from `$app/paths` will be replaced with relative asset paths during server-side rendering, resulting in portable HTML.
 		 * If `false`, `%sveltekit.assets%` and references to build artifacts will always be root-relative paths, unless `paths.assets` is an external URL
 		 *
+		 * [Single-page app](https://kit.svelte.dev/docs/single-page-apps) fallback pages will always use absolute paths, regardless of this setting.
+		 *
 		 * If your app uses a `<base>` element, you should set this to `false`, otherwise asset URLs will incorrectly be resolved against the `<base>` URL rather than the current page.
 		 * @default undefined
 		 */

packages/kit/types/index.d.ts

@@ -458,6 +458,8 @@ declare module '@sveltejs/kit' {
 			 * If `true`, `base` and `assets` imported from `$app/paths` will be replaced with relative asset paths during server-side rendering, resulting in portable HTML.
 			 * If `false`, `%sveltekit.assets%` and references to build artifacts will always be root-relative paths, unless `paths.assets` is an external URL
 			 *
+			 * [Single-page app](https://kit.svelte.dev/docs/single-page-apps) fallback pages will always use absolute paths, regardless of this setting.
+			 *
 			 * If your app uses a `<base>` element, you should set this to `false`, otherwise asset URLs will incorrectly be resolved against the `<base>` URL rather than the current page.
 			 * @default undefined
 			 */