Support explicit empty slot drop zones in shared editor

packages/block-editor/test/runtime.test.ts

@@ -1,7 +1,7 @@
 import { expect, test } from "bun:test";
 import { readFileSync } from "node:fs";
 import { Window } from "happy-dom";
-import type { DndManagerOptions, SortableRegistration } from "@wrnexus/dnd";
+import type { DndManagerOptions, SortableRegistration, DropZoneRegistration } from "@wrnexus/dnd";
 import type { MountedEditor, MountEditorOptions } from "../src/runtime.ts";
 
 const code = new Bun.Transpiler({ loader: "ts" })
@@ -19,13 +19,18 @@ function fixture(onMove: MountEditorOptions["onMove"] = () => false) {
   const backends: Array<{
     options: DndManagerOptions;
     registrations: SortableRegistration[];
+    zones: DropZoneRegistration[];
     destroyed: number;
   }> = [];
   const mount = new Function("createDndManager", code + ";return mountBlockEditor;")(
     (options: DndManagerOptions) => {
-      const backend = { options, registrations: [] as SortableRegistration[], destroyed: 0 };
+      const backend = { options, registrations: [] as SortableRegistration[], zones: [] as DropZoneRegistration[], destroyed: 0 };
       backends.push(backend);
       return {
+        registerDropZone: (registration: DropZoneRegistration) => {
+          backend.zones.push(registration);
+          return () => {};
+        },
         registerSortable: (registration: SortableRegistration) => {
           backend.registrations.push(registration);
           return () => {};
@@ -66,6 +71,28 @@ test("cancelled optimistic moves restore original DOM nodes across containers",
   f.editor.destroy();
 });
 
+test("empty named slot targets produce validated editor move intents and refresh away when populated", async () => {
+  const moves: unknown[] = [];
+  const f = fixture(intent => { moves.push(intent); return true; });
+  const target = f.document.createElement("div");
+  target.dataset.blockParent = "parent:empty";
+  target.dataset.blockSlotTarget = "content:empty";
+  f.root.append(target);
+  f.editor.refresh();
+  const backend = f.backends.at(-1)!;
+  expect(backend.zones).toHaveLength(1);
+  const zone = backend.zones[0]!;
+  expect(backend.options.accept!(["a"], { group: zone.group, index: 0 })).toBe(true);
+  await backend.options.onMoveIntent({ ids: ["a"], from: [], to: { group: zone.group, index: 0 } });
+  expect(moves).toEqual([{ nodeIds: ["a"], to: { parentId: "parent:empty", slot: "content:empty", index: 0 } }]);
+  const child = f.root.querySelector('[data-block-id="a"]')! as any;
+  child.dataset.blockParent = "parent:empty"; child.dataset.blockSlot = "content:empty";
+  target.append(child);
+  f.editor.refresh();
+  expect(f.backends.at(-1)!.zones).toHaveLength(0);
+  f.editor.destroy();
+});
+
 test("slot and parent identifiers containing colons round-trip without collisions", async () => {
   const moves: unknown[] = [];
   const f = fixture((intent) => {