Module drag 

Drag-and-drop protocol: Draggable sources, DropTarget targets, and DragPayload. Drag-and-drop protocol (bd-1csc.1 + bd-1csc.2).

Defines the Draggable trait for drag sources and DropTarget trait for drop targets, along with DragPayload for transferring typed data, DragState for tracking active drags, DropPosition for specifying where within a target the drop occurs, and DropResult for communicating drop outcomes.

Design

Integration with Semantic Events

Drag detection is handled by the gesture recognizer in ftui-core, which emits SemanticEvent::DragStart, DragMove, DragEnd, and DragCancel. The drag manager (bd-1csc.3) listens for these events, identifies the source widget via hit-test, and calls the Draggable methods.

Invariants

1. A drag operation is well-formed: exactly one DragStart followed by zero or more DragMove events, ending in either DragEnd or DragCancel.
2. on_drag_start is called exactly once per drag, before any DragMove.
3. on_drag_end is called exactly once per drag, with success = true if dropped on a valid target, false otherwise.
4. drag_type must return a stable string for the lifetime of the drag.

Failure Modes

Failure	Cause	Fallback
No hit-test match at drag start	Click outside any draggable	Drag not initiated
Payload decode failure	Type mismatch at drop target	Drop rejected
Focus loss mid-drag	Window deactivation	DragCancel emitted
Escape pressed mid-drag	User cancellation	DragCancel emitted (if cancel_on_escape)

Structs

DragConfig

Configuration for drag gesture detection.

DragPayload

Data carried during a drag operation.

DragPreview

Overlay widget that renders a drag preview at the cursor position.

DragPreviewConfig

Configuration for the drag preview overlay.

DragState

Active drag operation state.

Enums

DropPosition

Where within a drop target the drop will occur.

DropResult

Outcome of a drop operation.

Traits

Draggable

Trait for widgets that can be drag sources.

DropTarget

Trait for widgets that can accept drops.