fix(#4421): rescind #494's macOS full-matrix skip on changed test files (#4427)

scripts/ci-test-scope.cjs

@@ -526,15 +526,14 @@ function classify(files) {
 
     if (file.startsWith('tests/') && file.endsWith('.test.cjs')) {
       targeted.add(file);
-      // #494 invariant, narrowed: a changed test must still be exercised on
-      // the divergent OS before merge, but at per-file cost — it ALWAYS joins
-      // the scoped windows lane instead of triggering the three full parity
-      // lanes. (full_matrix fired on 15/15 sampled PRs because test-driven
-      // PRs always touch tests/, costing ~25 runner-minutes each.) Changed
-      // tests already run on the two ubuntu-24 lanes via targeted_tests; the
-      // residual macOS / windows cross-product is covered by the full
-      // matrix on every push to next.
       windows.add(file);
+      // #494 originally narrowed this to skip full_matrix for changed test
+      // files, on the theory that ubuntu targeted_tests + the scoped windows
+      // lane already covered them. Rescinded per #4421: PR #4384 landed a
+      // macOS-only regression on 2026-09-06 that stayed invisible pre-merge
+      // precisely because this carve-out suppressed the only macOS signal.
+      // The ~25-runner-minute cost on test-touching PRs is accepted.
+      fullMatrix = true;
     }
 
     for (const rule of RULES) {

tests/ci-test-scope.test.cjs

@@ -152,6 +152,8 @@ describe('ci-test-scope.cjs', () => {
     const result = scopeFor(['tests/run-tests-harness.test.cjs']);
     assert.strictEqual(result.code_changed, true);
     assert.ok(result.targeted_tests.includes('tests/run-tests-harness.test.cjs'));
+    assert.strictEqual(result.full_matrix, true,
+      'expected full_matrix=true for a changed test file (rescinded #494 carve-out, see #4421)');
   });
 
   test('installer-sensitive changes request full matrix and install tests', () => {
@@ -277,33 +279,37 @@ describe('ci-test-scope.cjs', () => {
   });
 });
 
-describe('ci-test-scope superset invariant (#494, narrowed)', () => {
+describe('ci-test-scope superset invariant (#494, rescinded by #4421)', () => {
-  // Facet A (narrowed): a changed test file no longer triggers the full
+  // Facet A: #494 originally narrowed this so a changed test file joined only
-  // parity matrix — instead it must ALWAYS run on the scoped windows lane,
+  // the scoped windows lane instead of triggering the full parity matrix.
-  // so OS-specific breakage in the changed test (the #482 class) is still
+  // Rescinded per #4421 (2026-09-06 RCA: PR #4384 shipped a macOS-only
-  // exercised pre-merge. Ubuntu 22/24 coverage comes via targeted_tests.
+  // regression invisible pre-merge because of exactly this carve-out) — a
-  test('A1: a changed test file joins the windows scoped lane without full_matrix', () => {
+  // changed test file now ALWAYS sets full_matrix=true, in addition to still
+  // joining the scoped windows lane.
+  test('A1: a changed test file joins the windows scoped lane AND triggers full_matrix', () => {
     const result = scopeFor(['tests/perf-317-context-monitor-fs.test.cjs']);
-    assert.strictEqual(result.full_matrix, false,
+    assert.strictEqual(result.full_matrix, true,
-      `expected full_matrix=false for a tests/**-only change, got: ${JSON.stringify(result)}`);
+      `expected full_matrix=true for a tests/**-only change (rescinded #494 carve-out, see #4421), got: ${JSON.stringify(result)}`);
     assert.ok(result.targeted_tests.includes('tests/perf-317-context-monitor-fs.test.cjs'),
       `expected the changed test in targeted_tests, got: ${JSON.stringify(result.targeted_tests)}`);
     assert.ok(result.windows_tests.includes('tests/perf-317-context-monitor-fs.test.cjs'),
       `expected the changed test in windows_tests, got: ${JSON.stringify(result.windows_tests)}`);
   });
 
-  test('A2: a changed test file with no windows hint still joins the windows lane', () => {
+  test('A2: a changed test file with no windows hint still joins the windows lane and triggers full_matrix', () => {
     // commands.test.cjs matches none of the WINDOWS_HINTS substrings — the
     // unconditional changed-test → windows lane rule must include it anyway.
     const result = scopeFor(['tests/commands.test.cjs']);
-    assert.strictEqual(result.full_matrix, false);
+    assert.strictEqual(result.full_matrix, true,
+      `expected full_matrix=true (rescinded #494 carve-out, see #4421), got: ${JSON.stringify(result)}`);
     assert.ok(result.windows_tests.includes('tests/commands.test.cjs'),
       `expected hint-less changed test in windows_tests, got: ${JSON.stringify(result.windows_tests)}`);
   });
 
-  test('A3: a deleted/nonexistent test path falls back to the unit token, no full_matrix', () => {
+  test('A3: a deleted/nonexistent test path falls back to the unit token, still triggers full_matrix', () => {
     const result = scopeFor(['tests/some-new.test.cjs']);
-    assert.strictEqual(result.full_matrix, false);
+    assert.strictEqual(result.full_matrix, true,
+      `expected full_matrix=true even for a nonexistent tests/*.test.cjs path — the matrix decision is made on the changed-file name, not on-disk existence (rescinded #494 carve-out, see #4421), got: ${JSON.stringify(result)}`);
     // The nonexistent file is filtered by existingTests(); with nothing left,
     // the #408 fallback applies so the targeted lane still runs something.
     assert.deepStrictEqual(result.targeted_tests, ['unit']);