Avoid repeated canvas scans during drag registration

packages/block-editor/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@wrnexus/block-editor",
-  "version": "0.1.7",
+  "version": "0.1.8",
   "type": "module",
   "description": "Hydrated accessible editor runtime for WRNexus block documents.",
   "exports": {

packages/block-editor/src/runtime.ts

@@ -119,38 +119,35 @@ export function mountBlockEditor(root: HTMLElement, options: MountEditorOptions)
         });
       },
     });
+    const elements = [...root.querySelectorAll<HTMLElement>(
+      "[data-block-id][data-block-parent][data-block-slot]",
+    )];
+    const groupSizes = new Map<string, number>();
+    const registrations = elements.map(element => {
+      const parentId = element.dataset.blockParent!;
+      const slot = element.dataset.blockSlot!;
+      const group = JSON.stringify([parentId, slot]);
+      const index = groupSizes.get(group) ?? 0;
+      groupSizes.set(group, index + 1);
+      groups.set(group, { parentId, slot });
+      return { element, group, index };
+    });
     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]);
+      if (groupSizes.has(group)) continue;
       groups.set(group, { parentId, slot });
       cleanups.push(manager.registerDropZone({ id: `wrn-slot:${group}`, group, index: 0, element }));
     }
-    for (const element of root.querySelectorAll<HTMLElement>(
+    for (const { element, group, index } of registrations) {
-      "[data-block-id][data-block-parent][data-block-slot]",
-    )) {
       const id = element.dataset.blockId!;
-      const parent = element.dataset.blockParent!;
-      const slot = element.dataset.blockSlot!;
-      const group = JSON.stringify([parent, slot]);
-      groups.set(group, { parentId: parent, slot });
-      const siblings = [
-        ...root.querySelectorAll<HTMLElement>(
-          "[data-block-id][data-block-parent][data-block-slot]",
-        ),
-      ].filter(
-        (candidate) =>
-          candidate.dataset.blockParent === parent && candidate.dataset.blockSlot === slot,
-      );
       const handle = element.querySelector<HTMLElement>("[data-block-handle]") ?? undefined;
       cleanups.push(
         manager.registerSortable({
           id,
           group,
-          index: siblings.indexOf(element),
+          index,
           element,
           handle,
           type: element.dataset.blockType,

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"]')!;