The tree could be built nested but never rearranged: `set_parent` existed,
with its cycle check and its tests, and nothing in the UI called it. A
collection created in the wrong place stayed there.
Each row is already a drop target, so it becomes a `DragArea` too —
wrapped at the instantiation site the way the grid's cells are, which
keeps the row's own TouchArea nested underneath and a click still
selecting. `allow-move`, not copy: a collection has one parent, unlike a
photograph, which is filed in as many collections as you like.
The drop is handed only the target's id, so the source is remembered from
the press that precedes the drag — Slint builds the payload through a
`pure` binding, which must not have side effects. An image drag always
fills `dragging`, so an empty payload with a remembered row is
unambiguously a rearrangement; the row is taken rather than read, or a
later empty drop would move a collection nobody touched.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
dtourolleandClaude Opus 5
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