Add test for "fetch redirect", add CSS value validation for external render (#37207) (#37216)

Backport #37207

web_src/js/features/common-page.ts

@@ -156,8 +156,8 @@ export function checkAppUrl() {
   if (curUrl.startsWith(appUrl) || `${curUrl}/` === appUrl) {
     return;
   }
-  showGlobalErrorMessage(`Your ROOT_URL in app.ini is "${appUrl}", it's unlikely matching the site you are visiting.
-Mismatched ROOT_URL config causes wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.`, 'warning');
+  showGlobalErrorMessage(`The detected web site URL is "${appUrl}", it's unlikely matching the site config.
+Mismatched app.ini ROOT_URL or reverse proxy "Host/X-Forwarded-Proto" config might cause wrong URL links for web UI/mail content/webhook notification/OAuth2 sign-in.`, 'warning');
 }
 
 export function checkAppUrlScheme() {