fix: preserve the document path in hash-router resolve

.changeset/hash-resolve-document.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: keep hash-router links on the current document when resolving paths

packages/kit/src/runtime/app/paths/client.js

@@ -39,10 +39,11 @@ export function asset(file) {
 	return (assets || base) + '/' + path;
 }
 
-const pathname_prefix = hash_routing ? '#' : '';
+const pathname_prefix = hash_routing ? '#' : base;
 
 /**
  * Resolve a pathname by prefixing it with the base path, if any, or resolve a route ID by populating dynamic segments with parameters.
+ * In hash routing mode, the returned URL starts with `#`.
  *
  * During server rendering, the base path is relative and depends on the page currently being rendered.
  *
@@ -73,12 +74,10 @@ export function resolve(...args) {
 			throw new Error(`Missing params for dynamic route ID ${id}`);
 		}
 
-		return (
-			/** @type {ResolvedPathname} */ (base + pathname_prefix + resolve_route(id, params ?? {}))
-		);
+		return /** @type {ResolvedPathname} */ (pathname_prefix + resolve_route(id, params ?? {}));
 	}
 
-	return /** @type {ResolvedPathname} */ (base + pathname_prefix + '/' + id);
+	return /** @type {ResolvedPathname} */ (pathname_prefix + '/' + id);
 }
 
 /**

packages/kit/src/runtime/app/paths/client.spec.js

@@ -0,0 +1,16 @@
+import { expect, test, vi } from 'vitest';
+
+vi.hoisted(() => {
+	vi.stubGlobal('__SVELTEKIT_PAYLOAD__', {});
+	vi.stubGlobal('__SVELTEKIT_PATHS_BASE__', '/base');
+	vi.stubGlobal('__SVELTEKIT_PATHS_ASSETS__', '');
+	vi.stubGlobal('__SVELTEKIT_APP_DIR__', '_app');
+	vi.stubGlobal('__SVELTEKIT_HASH_ROUTING__', true);
+});
+
+const { resolve } = await import('./client.js');
+
+test('hash routing resolves to a fragment without the base', () => {
+	expect(resolve('about')).toBe('#/about');
+	expect(resolve('/blog/[slug]', { slug: 'hello' })).toBe('#/blog/hello');
+});

packages/kit/types/index.d.ts

@@ -2923,6 +2923,7 @@ declare module '$app/paths' {
 	export function asset(file: AssetPath): string;
 	/**
 	 * Resolve a pathname by prefixing it with the base path, if any, or resolve a route ID by populating dynamic segments with parameters.
+	 * In hash routing mode, the returned URL starts with `#`.
 	 *
 	 * During server rendering, the base path is relative and depends on the page currently being rendered.
 	 *