slim sidebar - use sameSite lax by default

- this will ensure that the cookie will work as expected in future browser changes on HTTP or HTTPS setups
- fixes #7910#

client/src/components/Sidebar/index.tsx

@@ -7,6 +7,7 @@ import { Sidebar } from './Sidebar';
 export const SIDEBAR_COLLAPSED_COOKIE_NAME = 'wagtail_sidebar_collapsed';
 
 export function initSidebar() {
+  const cookieOptions = { sameSite: 'lax' };
   const element = document.getElementById('wagtail-sidebar');
   const rawProps = document.getElementById('wagtail-sidebar-props');
 
@@ -33,10 +34,10 @@ export function initSidebar() {
     const onExpandCollapse = (_collapsed: boolean) => {
       if (_collapsed) {
         document.body.classList.add('sidebar-collapsed');
-        Cookies.set(SIDEBAR_COLLAPSED_COOKIE_NAME, 1);
+        Cookies.set(SIDEBAR_COLLAPSED_COOKIE_NAME, 1, cookieOptions);
       } else {
         document.body.classList.remove('sidebar-collapsed');
-        Cookies.set(SIDEBAR_COLLAPSED_COOKIE_NAME, 0);
+        Cookies.set(SIDEBAR_COLLAPSED_COOKIE_NAME, 0, cookieOptions);
       }
     };