Support explicit empty slot drop zones in shared editor

packages/dnd/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@wrnexus/dnd",
-  "version": "0.1.2",
+  "version": "0.1.3",
   "type": "module",
   "description": "Stable accessible drag-and-drop primitives for WRNexus.",
   "exports": {

packages/dnd/src/manager.ts

@@ -1,4 +1,4 @@
-import { DragDropManager } from "@dnd-kit/dom";
+import { DragDropManager, Droppable } from "@dnd-kit/dom";
 import { Sortable } from "@dnd-kit/dom/sortable";
 import type {
   DndManager,
@@ -19,6 +19,7 @@ export function createDndManager(options: DndManagerOptions): DndManager {
     }
   >();
   const manager = new DragDropManager();
+  const zones = new Map<string | number, { droppable: Droppable; location: { group: string | number; index: number } }>();
   let activeIds: Array<string | number> = [];
   let generation = 0;
   let destroyed = false;
@@ -39,7 +40,7 @@ export function createDndManager(options: DndManagerOptions): DndManager {
     const target = event.operation.target as
       (typeof event.operation.target & { group?: string | number; index?: number }) | null;
     if (!target) return;
-    const destination = { group: target.group ?? "default", index: target.index ?? 0 };
+    const destination = zones.get(target.id)?.location ?? { group: target.group ?? "default", index: target.index ?? 0 };
     if (options.accept && !options.accept(activeIds, destination)) {
       event.preventDefault();
       options.announce?.({ type: "invalid", ids: activeIds, location: destination });
@@ -70,7 +71,7 @@ export function createDndManager(options: DndManagerOptions): DndManager {
           index: source.initialIndex ?? source.index ?? 0,
         },
       ],
-      to: { group: source.group ?? "default", index: source.index ?? 0 },
+      to: { ...(event.operation.target ? zones.get(event.operation.target.id)?.location : undefined) ?? { group: source.group ?? "default", index: source.index ?? 0 } },
     };
     // A pending commit must not own the next gesture's selection or lifecycle.
     activeIds = [];
@@ -96,9 +97,21 @@ export function createDndManager(options: DndManagerOptions): DndManager {
   });
 
   return {
+    registerDropZone(registration) {
+      if (destroyed) throw new Error("Cannot register a drop zone after manager destruction.");
+      if (entries.has(registration.id) || zones.has(registration.id)) throw new Error(`Duplicate draggable or drop zone id ${String(registration.id)}.`);
+      if (!Number.isSafeInteger(registration.index) || registration.index < 0) throw new Error("Drop zone index must be a non-negative safe integer.");
+      const droppable = new Droppable({ id: registration.id, element: registration.element, accept: registration.accepts, disabled: registration.disabled }, manager);
+      zones.set(registration.id, { droppable, location: { group: registration.group, index: registration.index } });
+      return () => {
+        if (zones.get(registration.id)?.droppable !== droppable) return;
+        droppable.destroy();
+        zones.delete(registration.id);
+      };
+    },
     registerSortable(registration) {
       if (destroyed) throw new Error("Cannot register a sortable after manager destruction.");
-      if (entries.has(registration.id))
+      if (entries.has(registration.id) || zones.has(registration.id))
         throw new Error(`Duplicate sortable id ${String(registration.id)}.`);
       const sortable = new Sortable(
         {
@@ -134,6 +147,8 @@ export function createDndManager(options: DndManagerOptions): DndManager {
       activeIds = [];
       for (const entry of entries.values()) entry.sortable.destroy();
       entries.clear();
+      for (const zone of zones.values()) zone.droppable.destroy();
+      zones.clear();
       manager.destroy();
     },
   };

packages/dnd/src/types.ts

@@ -40,6 +40,14 @@ export interface DndManagerOptions {
 }
 
 export interface DndManager {
+  registerDropZone(registration: DropZoneRegistration): () => void;
   registerSortable(registration: SortableRegistration): () => void;
   destroy(): void;
 }
+
+export interface DropZoneRegistration extends SortableLocation {
+  id: DndId;
+  element: HTMLElement;
+  accepts?: DndId[];
+  disabled?: boolean;
+}

packages/dnd/test/manager-lifecycle.test.ts

@@ -36,8 +36,9 @@ function fixture(overrides: Partial<DndManagerOptions> = {}) {
   const create = new Function(
     "DragDropManager",
     "Sortable",
+    "Droppable",
     compiled + ";return createDndManager;",
-  )(Backend, Sortable);
+  )(Backend, Sortable, Sortable);
   const manager: DndManager = create({
     onMoveIntent: () => true,
     announce: (event: DndAnnouncement) => announcements.push(event),
@@ -54,8 +55,8 @@ function fixture(overrides: Partial<DndManagerOptions> = {}) {
     announcements,
     instances,
     start: (id: string) => listeners.dragstart!(event(id)),
-    end: (id: string) => listeners.dragend!(event(id)),
-    over: (target: { group: string; index: number }) => {
+    end: (id: string, target?: { id: string }) => listeners.dragend!({ ...event(id), operation: { ...event(id).operation, target } }),
+    over: (target: { id?: string; group?: string; index?: number }) => {
       let prevented = false;
       listeners.dragover!({
         ...event("first"),
@@ -93,6 +94,34 @@ test("late drop resolution cannot clear or announce a newer gesture", async () =
   ]);
 });
 
+test("empty drop zones use explicit locations for acceptance and commit", async () => {
+  const commits: unknown[] = [];
+  const f = fixture({ accept: (_ids, to) => to.group === "empty", onMoveIntent: intent => { commits.push(intent.to); } });
+  const registration = { id: "zone", group: "empty", index: 0, element: {} as HTMLElement };
+  const cleanup = f.manager.registerDropZone(registration);
+  f.start("block");
+  expect(f.over({ id: "zone" })).toBe(false);
+  expect(f.announcements.at(-1)?.location).toEqual({ group: "empty", index: 0 });
+  await f.end("block", { id: "zone" });
+  expect(commits).toEqual([{ group: "empty", index: 0 }]);
+  expect(() => f.manager.registerSortable(registration)).toThrow("Duplicate");
+  cleanup();
+  f.manager.registerDropZone(registration);
+  cleanup();
+  expect(() => f.manager.registerDropZone(registration)).toThrow("Duplicate");
+  f.manager.destroy(); f.manager.destroy();
+  expect(f.instances.map(instance => instance.destroyed)).toEqual([1, 1]);
+  expect(() => f.manager.registerDropZone(registration)).toThrow("destruction");
+});
+
+test("invalid drop zone indices are rejected before backend registration", () => {
+  const f = fixture();
+  for (const index of [-1, 0.5, Infinity, Number.MAX_SAFE_INTEGER + 1]) {
+    expect(() => f.manager.registerDropZone({ id: "zone", group: "empty", index, element: {} as HTMLElement })).toThrow("index");
+  }
+  expect(f.instances).toEqual([]);
+});
+
 test("hover acceptance checks the proposed target instead of the source's old group", () => {
   const f = fixture({ accept: (_ids, to) => to.group === "allowed" });
   f.start("first");