Add shared canvas annotation and drag handle runtime

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

@@ -0,0 +1,37 @@
+import { expect, test } from "bun:test";
+import { readFileSync } from "node:fs";
+import { Window } from "happy-dom";
+
+const code = new Bun.Transpiler({ loader: "ts" }).transformSync(readFileSync(new URL("../src/canvas.ts", import.meta.url), "utf8"))
+  .replace(/^import .*;\s*$/gm, "").replace("export function mountBlockCanvas", "function mountBlockCanvas");
+
+test("canvas annotates exact node locations and cleans editor decorations without changing content", () => {
+  const window = new Window();
+  const root = window.document.body;
+  root.innerHTML = '<table data-block-id="root"><tbody><tr><td data-block-id="column" draggable="true"><table></table></td></tr></tbody></table>';
+  const original = root.innerHTML;
+  const tree = { id: "root", type: "layout.email-document", slots: { content: [{ id: "column", type: "layout.column", slots: { content: [] } }] } };
+  const before = JSON.stringify(tree);
+  let destroyed = 0;
+  const mount = new Function("mountBlockEditor", code + ";return mountBlockCanvas;")(() => ({ destroy: () => destroyed++, refresh() {} }));
+  const mounted = mount(root, tree, {});
+  const column = root.querySelector('[data-block-id="column"]')!;
+  expect(column.getAttribute("data-block-parent")).toBe("root");
+  expect(column.getAttribute("data-block-slot")).toBe("content");
+  expect(column.getAttribute("draggable")).toBe("false");
+  expect(root.querySelectorAll("[data-block-handle]")).toHaveLength(1);
+  expect(root.querySelector("[data-block-slot-target]")?.getAttribute("data-block-parent")).toBe("column");
+  expect(JSON.stringify(tree)).toBe(before);
+  mounted.destroy(); mounted.destroy();
+  expect(destroyed).toBe(1);
+  expect(root.innerHTML).toBe(original);
+});
+
+test("failed canvas mounting rolls back annotations", () => {
+  const root = new Window().document.body;
+  root.innerHTML = '<div data-block-id="root"><div data-block-id="child">Content</div></div>';
+  const before = root.innerHTML;
+  const mount = new Function("mountBlockEditor", code + ";return mountBlockCanvas;")(() => { throw new Error("backend unavailable"); });
+  expect(() => mount(root, { id: "root", type: "root", slots: { content: [{ id: "child", type: "text", slots: {} }] } }, {})).toThrow("backend unavailable");
+  expect(root.innerHTML).toBe(before);
+});