This is probably easiest to explain by example:
- Open a new document.
- Type some text, let’s say “A”.
- Click the triangle, or hit Command-9, to collapse the current row. Note that there’s no visible effect, the triangle doesn’t change.
- Hit enter twice, putting the cursor two rows down.
- Hit tab, indenting the current row.
- Type some text, let’s say “B”.
- Delete the second row.
This causes B to collapse into A, despite that A was rendered as if not collapsed. If one doesn’t remember collapsing A, maybe because it was a while ago, it seems that A collapsed on its own.
I think there needs to at least be some visual indication of what’s going on here, but what makes the most sense to me is to just say that rows with no children are never collapsed.