Support explicit empty slot drop zones in shared editor

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");