Merge pull request '#54: resilient undo E2E (preflight skip-guard) — fixes main E2E red' (#55) from s54-undo-e2e-resilience into main

`executeCommand("undo")` is non-functional in some headless `.vscode-test`
instances (it does not restore the buffer — see #54's diagnosis), which
false-fails every undo-dependent host E2E and turns `main`'s E2E red there even
though the product code is fine.

Add a RUNTIME preflight probe (`undoCapable.ts`): edit a scratch buffer, undo,
and report whether the buffer was actually restored (memoized per run). The #38
undoMarks suite gates on it via `suiteSetup` — running normally where undo works
(full coverage preserved, real regressions still caught) and skipping with a LOUD
console warning where it doesn't (no false red, no silent loss — the skip is
logged and shows as `pending`).

This fixes the "main E2E red" symptom. The underlying headless-undo limitation is
documented in #54; #40's undo-behavior coverage runs wherever undo works.

Test-infra only — no product code changed. Verified: with undo broken locally the
#38 suite skips (1 pending) and both E2E passes exit 0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/e2e/suite/undoCapable.ts

@@ -0,0 +1,41 @@
+import * as fs from "fs";
+import * as path from "path";
+import * as vscode from "vscode";
+
+/**
+ * #54: `vscode.commands.executeCommand("undo")` is non-functional in some headless
+ * `.vscode-test` instances (it does not restore the buffer), which false-fails
+ * every undo-dependent E2E (#38, #40) and makes `main` E2E red there. This is a
+ * RUNTIME probe: it performs a real edit-then-undo on a scratch buffer and reports
+ * whether undo actually restored it. Undo-dependent suites gate on it — running
+ * normally where undo works (real coverage), skipping with a loud warning where it
+ * doesn't (no false red, no silent loss — the skip is logged). Memoized per run.
+ */
+let cached: boolean | undefined;
+
+export async function undoWorks(): Promise<boolean> {
+  if (cached !== undefined) return cached;
+  const abs = path.join(process.env.E2E_WORKSPACE!, "docs/.undo-probe.md");
+  fs.mkdirSync(path.dirname(abs), { recursive: true });
+  fs.writeFileSync(abs, "undo probe baseline\n", "utf8");
+  const doc = await vscode.workspace.openTextDocument(vscode.Uri.file(abs));
+  await vscode.window.showTextDocument(doc);
+  await new Promise((r) => setTimeout(r, 250));
+  const edit = new vscode.WorkspaceEdit();
+  edit.insert(doc.uri, doc.positionAt(doc.getText().length), "PROBE-EDIT-MARKER");
+  await vscode.workspace.applyEdit(edit);
+  await new Promise((r) => setTimeout(r, 250));
+  if (!doc.getText().includes("PROBE-EDIT-MARKER")) {
+    cached = false; // even the edit didn't take — treat as not undo-capable
+    return cached;
+  }
+  await vscode.commands.executeCommand("undo");
+  await new Promise((r) => setTimeout(r, 250));
+  cached = !doc.getText().includes("PROBE-EDIT-MARKER"); // undo removed the marker → undo works
+  return cached;
+}
+
+/** Loud, single-line reason logged when an undo suite skips (no silent loss — #54). */
+export const UNDO_SKIP_REASON =
+  "[E2E] SKIPPING undo-dependent suite — executeCommand('undo') is non-functional in this " +
+  "VS Code test instance (see vscode-cowriting-plugin#54). These tests run where undo works.";

test/e2e/suite/undoMarks.test.ts

@@ -3,6 +3,7 @@ import * as fs from "fs";
 import * as path from "path";
 import * as vscode from "vscode";
 import type { CowritingApi } from "../../../src/extension";
+import { undoWorks, UNDO_SKIP_REASON } from "./undoCapable";
 
 const WS = process.env.E2E_WORKSPACE!;
 const settle = () => new Promise((r) => setTimeout(r, 400));
@@ -34,6 +35,14 @@ suite("F10 #38 — undo does not mis-attribute restored text (host E2E, no LLM)"
   const DOC_REL = "docs/undo38.md";
   const BASE = "Alpha bravo charlie.\n";
 
+  // #54: skip (loudly) where executeCommand("undo") is non-functional; run where it works.
+  suiteSetup(async function () {
+    if (!(await undoWorks())) {
+      console.warn(UNDO_SKIP_REASON);
+      this.skip();
+    }
+  });
+
   test("undo of a deletion of baseline text leaves it unattributed (not human)", async () => {
     const { doc, key } = await freshDoc(DOC_REL, BASE);
     const api = await getApi();