test(fake-dom): cover the four uncovered regions in the fake document

tests/fake-dom-harness.test.mjs

@@ -0,0 +1,77 @@
+// tests/tools/fake-dom.mjs is the stand-in document every focus-trap test
+// mounts against. The branches pinned here are the ones the focus-trap suite
+// never reaches, because that suite only ever registers one keydown listener
+// and only ever runs where no global document exists. A regression in them
+// would surface as a confusing failure in an unrelated suite, or as a leaked
+// global document that quietly changes how a later test behaves.
+
+import assert from 'node:assert/strict';
+import test from 'node:test';
+
+import { installFakeDom } from './tools/fake-dom.mjs';
+
+test('removing a listener for an unregistered type is a no-op', () => {
+  // A hook that tears down a listener type it never added must not throw:
+  // the fake document has to be as forgiving as the real one.
+  const dom = installFakeDom();
+  try {
+    assert.doesNotThrow(() =>
+      dom.document.removeEventListener('click', () => {}),
+    );
+
+    const handler = () => {};
+    dom.document.addEventListener('keydown', handler);
+    assert.equal(dom.keydownListenerCount(), 1);
+
+    // A handler that was never added leaves the registered one in place.
+    dom.document.removeEventListener('keydown', () => {});
+    assert.equal(dom.keydownListenerCount(), 1);
+
+    dom.document.removeEventListener('keydown', handler);
+    assert.equal(dom.keydownListenerCount(), 0);
+  } finally {
+    dom.restore();
+  }
+});
+
+test('a document with no keydown listener counts and dispatches as empty', () => {
+  // Suites assert keydownListenerCount() === 0 after unmount to prove the
+  // trap detached. If the count threw, or dispatching crashed, on a document
+  // that never saw a listener, that assertion could never be made.
+  const dom = installFakeDom();
+  try {
+    assert.equal(dom.keydownListenerCount(), 0);
+    assert.deepEqual(dom.dispatchKeyDown({ key: 'Escape' }), {
+      defaultPrevented: false,
+    });
+  } finally {
+    dom.restore();
+  }
+});
+
+test('restore puts back a document that existed before the install', () => {
+  // Under a DOM-providing runner globalThis.document is already set. restore()
+  // must hand that object back rather than delete the global, or every test
+  // that ran after the first fake install would see no document at all.
+  const previous = { marker: 'pre-existing' };
+  globalThis.document = previous;
+  try {
+    const dom = installFakeDom();
+    assert.notEqual(globalThis.document, previous);
+    assert.equal(globalThis.document, dom.document);
+
+    dom.restore();
+    assert.equal(globalThis.document, previous);
+    assert.ok('document' in globalThis);
+  } finally {
+    delete globalThis.document;
+  }
+});
+
+test('restore deletes the global when no document existed before', () => {
+  assert.equal('document' in globalThis, false);
+  const dom = installFakeDom();
+  assert.equal(globalThis.document, dom.document);
+  dom.restore();
+  assert.equal('document' in globalThis, false);
+});