test(f7): host E2E for the track-changes preview (#21)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/e2e/fixtures/workspace/docs/notes.txt

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+Plain text, not markdown — F7 must refuse this (F6 covers it instead).

test/e2e/fixtures/workspace/docs/preview.md

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+# Preview fixture
+
+A target sentence Claude will rewrite via the seam.
+
+Some stable prose that does not change during the test run.
+
+```mermaid
+flowchart LR
+  a --> b
+```
+
+```ts
+const stable = true;
+```

test/e2e/suite/trackChangesPreview.test.ts

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+import * as assert from "assert";
+import * as path from "path";
+import * as vscode from "vscode";
+import type { CowritingApi } from "../../../src/extension";
+
+const WS = process.env.E2E_WORKSPACE!;
+const DOC_REL = "docs/preview.md";
+const docUri = () => vscode.Uri.file(path.join(WS, DOC_REL)).toString();
+
+async function openDoc(rel = DOC_REL): Promise<vscode.TextDocument> {
+  const uri = vscode.Uri.file(path.join(WS, rel));
+  const doc = await vscode.workspace.openTextDocument(uri);
+  await vscode.window.showTextDocument(doc);
+  return doc;
+}
+async function getApi(): Promise<CowritingApi> {
+  const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
+  const api = (await ext.activate()) as CowritingApi;
+  assert.ok(api?.trackChangesPreviewController, "extension exports trackChangesPreviewController");
+  return api;
+}
+const settle = () => new Promise((r) => setTimeout(r, 400));
+const kinds = (api: CowritingApi) =>
+  (api.trackChangesPreviewController.getLastModel(docUri()) ?? []).map((o) => o.kind);
+
+// Order-dependent (F2–F6 pattern): later tests consume earlier state.
+suite("F7 track-changes preview (host E2E — markdown only, programmatic seam, no LLM)", () => {
+  const TARGET = "A target sentence Claude will rewrite via the seam.";
+  const REPLACEMENT = "A SENTENCE CLAUDE REWROTE via the seam.";
+
+  test("open on a markdown doc → panel open, opened baseline shows no changes (PUC-1)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    assert.strictEqual(api.trackChangesPreviewController.isOpen(docUri()), false, "no panel yet");
+    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
+    await settle();
+    assert.strictEqual(api.trackChangesPreviewController.isOpen(docUri()), true, "panel open");
+    const model = api.trackChangesPreviewController.getLastModel(docUri());
+    assert.ok(model && model.length > 0, "a model was computed");
+    assert.ok(
+      model!.every((o) => o.kind === "unchanged"),
+      "baseline == buffer at open → every block unchanged",
+    );
+    void doc;
+  });
+
+  test("typing produces a changed/added block (PUC-2)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    const edit = new vscode.WorkspaceEdit();
+    const end = doc.positionAt(doc.getText().length);
+    edit.insert(doc.uri, end, "\n\nA freshly typed paragraph.\n");
+    assert.ok(await vscode.workspace.applyEdit(edit), "operator edit applied");
+    await settle();
+    assert.ok(
+      kinds(api).some((k) => k === "added" || k === "changed"),
+      "an added/changed block appears after typing",
+    );
+  });
+
+  test("accepting a proposal advances the baseline; the landed block goes unchanged (PUC-3, INV-18)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    const start = doc.getText().indexOf(TARGET);
+    assert.ok(start >= 0, "fixture contains the target sentence");
+    const id = await vscode.commands.executeCommand<string>("cowriting.proposeAgentEdit", {
+      uri: doc.uri.toString(),
+      start,
+      end: start + TARGET.length,
+      newText: REPLACEMENT,
+      model: "sonnet",
+      sessionId: "e2e-f7",
+      turnId: "turn-f7-1",
+    });
+    assert.ok(id, "propose returns an id");
+    assert.ok(await api.proposalController.acceptById(DOC_REL, id!), "accept applies via the seam");
+    await settle();
+    // The baseline advanced to include REPLACEMENT, so the buffer == baseline for
+    // that block → it is NOT marked. Confirm the replacement is not flagged as a change.
+    const model = api.trackChangesPreviewController.getLastModel(docUri()) ?? [];
+    const replacementMarked = model.some(
+      (o) => o.kind !== "unchanged" && o.block.raw.includes("CLAUDE REWROTE"),
+    );
+    assert.ok(!replacementMarked, "the just-landed text renders unmarked (baseline advanced)");
+  });
+
+  test("pin resets the baseline to now → preview shows no marks (PUC-4)", async () => {
+    const doc = await openDoc();
+    const api = await getApi();
+    await vscode.commands.executeCommand("cowriting.pinDiffBaseline");
+    await settle();
+    assert.ok(
+      kinds(api).every((k) => k === "unchanged"),
+      "after pin, every block is unchanged (baseline == buffer)",
+    );
+    void doc;
+  });
+
+  test("a non-markdown doc → command warns and opens no panel (PUC-6, markdown-only)", async () => {
+    const txt = await openDoc("docs/notes.txt");
+    const api = await getApi();
+    const key = txt.uri.toString();
+    assert.notStrictEqual(txt.languageId, "markdown", "fixture is not markdown");
+    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
+    await settle();
+    assert.strictEqual(api.trackChangesPreviewController.isOpen(key), false, "no panel for non-markdown");
+  });
+});