Expand comment [skip ci].

packages/ckeditor5-mention/src/mentioncommand.ts

@@ -191,7 +191,9 @@ export default class MentionCommand extends Command {
 				isInsertedInBrackets = isPrecededByOpeningBracket && isFollowedByBracketClosure;
 			}
 
-			// Don't add a white space if there's already one after the mention or if the mention was inserted in brackets.
+			// Don't add a white space if either of the following is true:
+			// * there's already one after the mention;
+			// * the mention was inserted in the empty matching brackets.
 			// https://github.com/ckeditor/ckeditor5/issues/4651
 			if ( !isInsertedInBrackets && !isFollowedByWhiteSpace ) {
 				model.insertContent( writer.createText( ' ', currentAttributes ), range!.start.getShiftedBy( mentionText.length ) );