Thanks so much for getting to the bottom of this (it was news to me), though I think the way you described it is not precisely correct so let me restate:
- Dragging a subtask onto a section highlights the section title’s rectangle as the drop target; dropping there makes that subtask a member of that project/section (the subtask remains a subtask of its parent as well so you see the subtask in both places)
- Dragging a subtask elsewhere in the tasks list shows a horizontal rule as the drop target; dropping there disconnects that subtask from its parent and maks it a top-level task as a member of that project/section
The primary difference from the way you characterized it is that it’s not actually dependent on whether the section is empty; it’s where you choose to drop–on a section (empty or not empty) title, or not on a section title.
Note a related behavior: If you drag and drop a subtask onto a project name in the left sidebar, it moves the subtask to the top of that project (like behavior 2 above).
@Marie, Can you explain what the thinking is behind this? It might be handy once one knows about it, but the UX problems are 1) it’s unexpected behavior (as Shawn and I confirmed), and 2) it’s undiscoverable (as Shawn and I confirmed).