Gerda—character from Disney Frozen—was a rich editor for the Arendelle Language. In trying to have intelligent autocompletion for the language (this was way before Language Server Protocol and few languages actually had this) so both an Arendelle O(n) parser to give me fast auto completion list was needed as well as a graphical text editor with auto completion on type.

Another screenshot of the Gerda that showcases its query algorithm. (Arendelle Language)