fix: handle properly adapter fallback

src/config.ts

@@ -58,27 +58,32 @@ export function configureSvelteKitOptions(
 
 function createManifestTransform(base: string, options?: KitOptions): ManifestTransform {
   return async (entries) => {
+    const defaultAdapterFallback = 'prerendered/fallback.html'
     const suffix = options?.trailingSlash === 'always' ? '/' : ''
     let adapterFallback = options?.adapterFallback
     let excludeFallback = false
     // the fallback will be always generated by SvelteKit.
     // The adapter will copy the fallback only if it is provided in its options: we need to exclude it
     if (!adapterFallback) {
-      adapterFallback = 'prerendered/fallback.html'
+      adapterFallback = defaultAdapterFallback
       excludeFallback = true
     }
 
-    const manifest = entries.filter(({ url }) => !(excludeFallback && url === adapterFallback)).map((e) => {
+    // the fallback will be always in .svelte-kit/output/prerendered/fallback.html
+    const manifest = entries.filter(({ url }) => !(excludeFallback && url === defaultAdapterFallback)).map((e) => {
       let url = e.url
       // client assets in `.svelte-kit/output/client` folder.
       // SSG pages in `.svelte-kit/output/prerendered/pages` folder.
       // fallback page in `.svelte-kit/output/prerendered` folder (fallback.html is the default).
-      if (url.startsWith('client/'))
+      if (url.startsWith('client/')) {
         url = url.slice(7)
-      else if (url.startsWith('prerendered/pages/'))
+      }
+      else if (url.startsWith('prerendered/pages/')) {
         url = url.slice(18)
-      else if (url.startsWith('prerendered/'))
-        url = url.slice(12)
+      }
+      else if (url === defaultAdapterFallback) {
+        url = adapterFallback!
+      }
 
       if (url.endsWith('.html')) {
         if (url.startsWith('/'))