fix: final-review wave — preview host INV-10 gate, thread status+offer contextValue tokens, establish-on-open baseline, polish

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/e2e/suite/baselineRouter.test.ts

@@ -6,6 +6,8 @@ import * as path from "node:path";
 import * as vscode from "vscode";
 import { activateApi, settle, settleUntil } from "./helpers";
 
+const WS = process.env.E2E_WORKSPACE!;
+
 suite("baseline router (PUC-1, INV-7/D13/D14)", () => {
   test("snapshot mode: enter captures; markReviewed re-pins clean", async () => {
     const api = await activateApi();
@@ -49,4 +51,37 @@ suite("baseline router (PUC-1, INV-7/D13/D14)", () => {
     );
     assert.strictEqual(api.diffViewController.getBaseline(doc.uri.toString())?.reason, "head");
   });
+
+  // Finding 3 (final whole-branch review, session native-surfaces-exec):
+  // DiffViewController only establishes a baseline on the registry's ENTER
+  // transition (a doc it can find already open) and, at startup, for docs
+  // already open at that moment. A registry member entered while its document
+  // was NOT yet open (the real-world case: a persisted coediting set restored
+  // after a window reload, whose member is only lazily opened later) used to
+  // never get a baseline at all — reproduced here directly via
+  // `coeditingRegistry.enter(uri)` on a URI with no open document yet (the
+  // registry API doesn't require one), then a genuine later open.
+  test("a registry member entered before its doc was open gets its baseline established once the doc IS opened", async () => {
+    const api = await activateApi();
+    const rel = "docs/finding3-establish-on-open.md";
+    const abs = path.join(WS, rel);
+    const content = "# late open\n\nestablish-on-open should catch this.\n";
+    fs.writeFileSync(abs, content, "utf8");
+    const uri = vscode.Uri.file(abs);
+
+    // Membership without an open document — DiffViewController's enter-
+    // transition handler no-ops (`textDocuments.find` misses), reproducing
+    // the gap: a coediting member with no mode/baseline yet.
+    api.coeditingRegistry.enter(uri);
+    assert.strictEqual(api.coeditingRegistry.isCoediting(uri), true, "sanity: registry membership recorded");
+    assert.strictEqual(api.diffViewController.modeOf(uri.toString()), undefined, "sanity: the gap is reproduced");
+
+    // A genuine later open — the fix's `renderIfOpen`/`onDidOpenTextDocument`
+    // path must establish the baseline it missed.
+    const doc = await vscode.workspace.openTextDocument(uri);
+    await vscode.window.showTextDocument(doc);
+    await settleUntil(() => api.diffViewController.modeOf(uri.toString()) !== undefined, 10000);
+    assert.strictEqual(api.diffViewController.modeOf(uri.toString()), "snapshot");
+    assert.strictEqual(api.diffViewController.getBaseline(uri.toString())?.text, content);
+  });
 });