Upgrade to latest Svelte version breaks mode-watcher

[Lexumiii]

Describe the bug

After upgrading to svelte 5.46.1 this error occures:

Uncaught (in promise) TypeError: render_fn is not a function

	in <unknown>
	in mode-watcher-full.svelte
	in mode-watcher.svelte
	in +layout.svelte
	in root.svelte

    at chunk-CKSF4M2J.js?v=35da2a1f:1874:17
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at block (chunk-FD5WVBO5.js?v=35da2a1f:3024:17)
    at head (chunk-CKSF4M2J.js?v=35da2a1f:1874:5)
    at Mode_watcher_full (mode-watcher.js?v=35da2a1f:1201:3)
    at chunk-CKSF4M2J.js?v=35da2a1f:306:58
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at branch (chunk-FD5WVBO5.js?v=35da2a1f:3038:10)
    at chunk-CKSF4M2J.js?v=35da2a1f:300:17
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at block (chunk-FD5WVBO5.js?v=35da2a1f:3024:17)
    at wrapper (chunk-CKSF4M2J.js?v=35da2a1f:292:5)
    at add_svelte_meta.componentTag (mode-watcher.js?v=35da2a1f:1318:15)
    at add_svelte_meta (chunk-FD5WVBO5.js?v=35da2a1f:545:12)

Reproduction

Dont think one is needed. Also the template for StackBlitz does not exist anymore..

Logs

Uncaught (in promise) TypeError: render_fn is not a function

	in <unknown>
	in mode-watcher-full.svelte
	in mode-watcher.svelte
	in +layout.svelte
	in root.svelte

    at chunk-CKSF4M2J.js?v=35da2a1f:1874:17
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at block (chunk-FD5WVBO5.js?v=35da2a1f:3024:17)
    at head (chunk-CKSF4M2J.js?v=35da2a1f:1874:5)
    at Mode_watcher_full (mode-watcher.js?v=35da2a1f:1201:3)
    at chunk-CKSF4M2J.js?v=35da2a1f:306:58
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at branch (chunk-FD5WVBO5.js?v=35da2a1f:3038:10)
    at chunk-CKSF4M2J.js?v=35da2a1f:300:17
    at update_reaction (chunk-FD5WVBO5.js?v=35da2a1f:3374:18)
    at update_effect (chunk-FD5WVBO5.js?v=35da2a1f:3502:21)
    at create_effect (chunk-FD5WVBO5.js?v=35da2a1f:2862:7)
    at block (chunk-FD5WVBO5.js?v=35da2a1f:3024:17)
    at wrapper (chunk-CKSF4M2J.js?v=35da2a1f:292:5)
    at add_svelte_meta.componentTag (mode-watcher.js?v=35da2a1f:1318:15)
    at add_svelte_meta (chunk-FD5WVBO5.js?v=35da2a1f:545:12)

System Info

System:
    OS: macOS 26.3
    CPU: (8) arm64 Apple M2
    Memory: 89.09 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 25.2.1 - /opt/homebrew/bin/node
    npm: 11.6.2 - /opt/homebrew/bin/npm
    bun: 1.2.9 - /opt/homebrew/bin/bun
    Deno: 2.1.9 - /opt/homebrew/bin/deno
  Browsers:
    Chrome: 123.0.6312.87
    Safari: 26.3
  npmPackages:
    @sveltejs/kit: 2.49.3 => 2.49.3 
    mode-watcher: ^1.1.0 => 1.1.0 
    svelte: 5.46.1 => 5.46.1

Severity

blocking all usage of mode-watcher

[Lexumiii]

Patchfile for anyone needing a quick fix:

diff --git a/node_modules/mode-watcher/dist/components/mode-watcher-full.svelte b/node_modules/mode-watcher/dist/components/mode-watcher-full.svelte
index fa7c24c..c6312a7 100644
--- a/node_modules/mode-watcher/dist/components/mode-watcher-full.svelte
+++ b/node_modules/mode-watcher/dist/components/mode-watcher-full.svelte
@@ -11,6 +11,9 @@
 		initConfig: Parameters<typeof setInitialMode>[0];
 		themeColors: ThemeColors;
 	} = $props();
+
+	// Svelte 5 compatible: build script tag as HTML string
+	const scriptTag = `<script${trueNonce ? ` nonce="${trueNonce}"` : ""}>(${setInitialMode.toString()})(${JSON.stringify(initConfig)});<\/script>`;
 </script>
 
 <svelte:head>
@@ -20,10 +23,8 @@
 		<!-- but that snippet does not run in vitest -->
 		<meta name="theme-color" content={themeColors.dark} />
 	{/if}
-	<!-- eslint-disable-next-line svelte/no-at-html-tags, prefer-template, svelte/no-unused-svelte-ignore --><!-- svelte-ignore hydration_html_changed -->
-	{@html `<script${trueNonce ? ` nonce=${trueNonce}` : ""}>(` +
-		setInitialMode.toString() +
-		`)(` +
-		JSON.stringify(initConfig) +
-		`);</script>`}
+	<!-- Svelte 5: Render script tag via {@html} outside of svelte:head -->
 </svelte:head>
+
+<!-- Inject script in body (will be moved to head by browser) -->
+{@html scriptTag}

[huntabyte]

No breaking changes should be introduced in a minor or patch svelte release, so please raise this issue with sveltejs/svelte, thank you!