[fix] Throw on invalid browser options (#6086)

* [fix] Throw on invalid browser options

Throw error if browser.hydrate is false and brower.router is true. The router needs at least the router component to be hydrated, and we currently can't dynamically compile Svelte components as hydratable or not on case-by-case basis
Closes #4382

* typo

Author: dummdidumm

.changeset/ten-gorillas-itch.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Throw error if browser.hydrate is false and browser.router is true

packages/kit/src/core/config/index.spec.js

@@ -246,6 +246,19 @@ test('fails if kit.appDir ends with slash', () => {
 	}, /^config\.kit\.appDir cannot start or end with '\/'. See https:\/\/kit\.svelte\.dev\/docs\/configuration$/);
 });
 
+test('fails if browser.hydrate is false and browser.router is true', () => {
+	assert.throws(() => {
+		validate_config({
+			kit: {
+				browser: {
+					hydrate: false,
+					router: true
+				}
+			}
+		});
+	}, /^config\.kit\.browser\.router cannot be true if config\.kit\.browser\.hydrate is false$/);
+});
+
 test('fails if paths.base is not root-relative', () => {
 	assert.throws(() => {
 		validate_config({

packages/kit/src/core/config/options.js

@@ -107,9 +107,14 @@ const options = object(
 				return input;
 			}),
 
-			browser: object({
-				hydrate: boolean(true),
-				router: boolean(true)
+			browser: validate({ hydrate: true, router: true }, (input, keypath) => {
+				const value = object({ hydrate: boolean(true), router: boolean(true) })(input, keypath);
+				if (!value.hydrate && value.router) {
+					throw new Error(
+						'config.kit.browser.router cannot be true if config.kit.browser.hydrate is false'
+					);
+				}
+				return value;
 			}),
 
 			csp: object({