Reduce discrepancies between server and client-side character count (#5360)

app/javascript/mastodon/features/compose/util/counter.js

@@ -5,5 +5,5 @@ const urlPlaceholder = 'xxxxxxxxxxxxxxxxxxxxxxx';
 export function countableText(inputText) {
   return inputText
     .replace(urlRegex, urlPlaceholder)
-    .replace(/(?:^|[^\/\w])@(([a-z0-9_]+)@[a-z0-9\.\-]+)/ig, '@$2');
+    .replace(/(?:^|[^\/\w])@(([a-z0-9_]+)@[a-z0-9\.\-]+[a-z0-9]+)/ig, '@$2');
 };