- Xml::input( 'user-reason', 60, $this->getRequest()->getVal( 'user-reason', false ),
- [ 'id' => 'wpReason', 'maxlength' => 255 ] ) .
+ Xml::input( 'user-reason', 60, $this->getRequest()->getVal( 'user-reason', false ), [
+ 'id' => 'wpReason',
+ // HTML maxlength uses "UTF-16 code units", which means that characters outside BMP
+ // (e.g. emojis) count for two each. This limit is overridden in JS to instead count
+ // Unicode codepoints (or 255 UTF-8 bytes for old schema).
+ 'maxlength' => $oldCommentSchema ? 255 : CommentStore::COMMENT_CHARACTER_LIMIT,
+ ] ) .