Drag-and-drop reordering

DragDrop::Reorder coordinates pointer and keyboard reordering without owning the application’s collection. It identifies items by stable IDs and reports a DragDrop::Target whose position is before, after, or inside.

reorder = Zaniah::DragDrop::Reorder.new(
  threshold: 4,
  locate: ->(point, source_id) { target_at(point, excluding: source_id) },
  keyboard: ->(id, direction) { adjacent_target(id, direction) },
  label: ->(id) { item_label(id) }
).on_drop do |event|
  move_item(event.source_id, event.target.id, event.target.position)
end

row
  .on_mouse_down { |event, _| reorder.press(row_id, event.position) }
  .on_drag { |event, _| reorder.move(event.position) }
  .on_mouse_up { |event, _| reorder.release(event.position) }
  .focusable(context: {reorderable: true}) { |action| reorder.action(row_id, action) }

The pointer locator is called only after the drag threshold is crossed. Virtual lists can derive a row with List::HeightIndex#index_at; uniform lists can use division by row height. Tree views can resolve a visible segment and return an inside target only for containers. No API requires enumerating or rendering the whole collection. Return nil for collection-specific no-ops, such as placing an item after its current predecessor.

Listen with on_event for press, start, enter, over, leave, drop, and cancel. Call remove(id) when application data removes a source or current target, and call cancel when an enclosing interaction ends. Callback failures reset transient drag state before being re-raised. Callbacks may update the application collection, but must not re-enter their Reorder controller; an attempt raises Zaniah::Error and cancels the active operation.

For keyboard access, mark the focused element with reorderable: true. Alt+Up and Alt+Down dispatch reorder_before and reorder_after; Escape dispatches cancel_reorder. accessibility_actions(id) returns the actions a containing item should expose, and action(id, action) routes them without pointer geometry. accessibility_node exposes the last result as a stable, atomic polite status node. Include that node in the containing component’s accessibility children, or forward on_announce to its existing live region. Native file drops are separate and are not handled by this primitive. The controller belongs to its UI owner: call it on that owner’s event thread and cancel before the owner is discarded.