State changes when inserting return are now in sync with other characters.
PIEDOKYQWPQIQV3UDNLHZU7BXDPI7YBZ2LOQ2I6MN5JQM2CPPDOAC
* Some ADT/interface support would be helpful in keeping per-line state in
sync. Any change to line data should clear line `fragments` and
`screen_line_starting_pos`.