Vocabulary hierarchy was always changed to single after term edit. This been fixed in #762604: Taxonomy terms list broken, term add/edit forgets it's parent
But we still has no test for "flatness" and this issue fixes it with more clear/readable condition in code
- Create a new vocabulary, on the term list page, you see "flat vocabulary"
- Add any term, on the term list page, the vocabulary type changed to "single hierarchy"
Expected behavior: the vocabulary type should stay in "flat vocabulary" as long as there is no parent/child relationship in the vocabulary.