Avoid repeated canvas scans during drag registration

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

@@ -74,6 +74,38 @@ function fixture(onMove: MountEditorOptions["onMove"] = () => false) {
   };
 }
 
+test("large canvas registration scans blocks once and preserves interleaved slot order", () => {
+  const f = fixture();
+  f.root.replaceChildren();
+  for (let i = 0; i < 300; i++) {
+    const node = f.document.createElement("div");
+    node.dataset.blockId = String(i);
+    node.dataset.blockParent = `parent:${i % 3}`;
+    node.dataset.blockSlot = "content";
+    f.root.append(node);
+  }
+  for (let i = 0; i < 4; i++) {
+    const slot = f.document.createElement("div");
+    slot.dataset.blockParent = `parent:${i}`;
+    slot.dataset.blockSlotTarget = "content";
+    f.root.append(slot);
+  }
+  const query = f.root.querySelectorAll.bind(f.root);
+  let scans = 0;
+  f.root.querySelectorAll = ((selector: string) => {
+    if (selector === "[data-block-id][data-block-parent][data-block-slot]") scans++;
+    return query(selector);
+  }) as typeof f.root.querySelectorAll;
+  f.editor.refresh();
+  const backend = f.backends.at(-1)!;
+  expect(scans).toBe(1);
+  expect(backend.registrations.map(({ id, group, index }) => ({ id, group, index }))).toEqual(
+    Array.from({ length: 300 }, (_, i) => ({ id: String(i), group: JSON.stringify([`parent:${i % 3}`, "content"]), index: Math.floor(i / 3) })),
+  );
+  expect(backend.zones.map(zone => zone.group)).toEqual([JSON.stringify(["parent:3", "content"])]);
+  f.editor.destroy();
+});
+
 test("cancelled optimistic moves restore original DOM nodes across containers", async () => {
   const f = fixture();
   const a = f.root.querySelector('[data-block-id="a"]')!;