duplicate cookies in .html case

packages/kit/src/runtime/server/cookie.js

@@ -1,5 +1,5 @@
 import { parse, serialize } from 'cookie';
-import { normalize_path, resolve } from '../../utils/url.js';
+import { add_data_suffix, normalize_path, resolve } from '../../utils/url.js';
 
 /**
  * Tracks all cookies set during dev mode so we can emit warnings
@@ -245,6 +245,14 @@ export function add_cookies_to_headers(headers, cookies) {
 	for (const new_cookie of cookies) {
 		const { name, value, options } = new_cookie;
 		headers.append('set-cookie', serialize(name, value, options));
+
+		// special case — for routes ending with .html, the route data lives in a sibling
+		// `.html__data.json` file rather than a child `/__data.json` file, which means
+		// we need to duplicate the cookie
+		if (options.path.endsWith('.html')) {
+			const path = add_data_suffix(options.path);
+			headers.append('set-cookie', serialize(name, value, { ...options, path }));
+		}
 	}
 }