Support explicit empty slot drop zones in shared editor

packages/block-editor/src/runtime.ts

@@ -107,6 +107,16 @@ export function mountBlockEditor(root: HTMLElement, options: MountEditorOptions)
         });
       },
     });
+    for (const element of root.querySelectorAll<HTMLElement>("[data-block-slot-target][data-block-parent]")) {
+      const parentId = element.dataset.blockParent!;
+      const slot = element.dataset.blockSlotTarget!;
+      const hasChildren = [...root.querySelectorAll<HTMLElement>("[data-block-id][data-block-parent][data-block-slot]")]
+        .some(child => child.dataset.blockParent === parentId && child.dataset.blockSlot === slot);
+      if (hasChildren) continue;
+      const group = JSON.stringify([parentId, slot]);
+      groups.set(group, { parentId, slot });
+      cleanups.push(manager.registerDropZone({ id: `wrn-slot:${group}`, group, index: 0, element }));
+    }
     for (const element of root.querySelectorAll<HTMLElement>(
       "[data-block-id][data-block-parent][data-block-slot]",
     )) {