feat(studio): a host undo key paints a still-saving move back at once (#4894)

* feat(studio): let a host's undo key paint a still-saving move back at once

* test(studio): prove the public revert from a .ts test and drop the export comment

* test(studio): a failing revert check cannot leave an edit pending, and a second revert paints nothing

* test(studio): load the package index once at collection, not inside the revert test

packages/studio/src/index.ts

@@ -148,6 +148,7 @@ export type {
 // A host's own waitForPendingDomEditSaves must also call this, or undo/redo
 // can race a write still in flight (see useTrackPendingTimelineEdit.ts).
 export { flushStudioPendingEdits } from "./utils/studioPendingEdits";
+export { revertNewestStudioPendingEdit } from "./utils/studioPendingEdits";
 export type { StudioPendingEditsDrainResult } from "./utils/studioPendingEdits";
 export type {
   CanEditTimelineElement,

packages/studio/src/utils/studioPendingEdits.test.ts

@@ -10,6 +10,7 @@ import {
   trackedStudioEdit,
 } from "./studioPendingEdits";
 import { StudioFileConflictError } from "./studioSaveDiagnostics";
+import { revertNewestStudioPendingEdit as hostRevert } from "../index";
 
 describe("studio pending edit flush", () => {
   it("waits for mounted panels to persist pending local edits", async () => {
@@ -277,3 +278,20 @@ describe("a pending edit whose start throws", () => {
     expect(hasStudioPendingEdits()).toBe(false);
   });
 });
+
+describe("the package's public revert", () => {
+  it("lets a host's own undo key paint a still-saving move back at once", async () => {
+    let left = "120px";
+    const edit = beginStudioPendingEdit(() => ((left = "0px"), () => void (left = "120px")));
+    try {
+      const putBack = hostRevert();
+      expect(left).toBe("0px");
+      expect(hostRevert()).toBeNull();
+      putBack?.();
+      expect(left).toBe("120px");
+    } finally {
+      edit.settle();
+      await flushStudioPendingEdits();
+    }
+  });
+});