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+// tests/tools/e2e-data-fixture-loader.cjs is the webpack loader
+// docusaurus.config.js installs on data/ when E2E_COVERAGE=1, and it is the
+// single point every data overlay in tests/e2e/fixtures/** passes through on
+// its way into the end-to-end coverage build.
+//
+// Nothing imported it. `grep -rl e2e-data-fixture-loader` over the repository
+// returns docusaurus.config.js (which names the path),
+// tests/tools/e2e-data-fixtures.cjs (a prose reference) and
+// tests/site-config.test.mjs:327, which asserts the config string and never
+// loads the module. The coverage reporter therefore has no row for it at all:
+// a file no test imports is recorded nowhere, so its lines are absent from the
+// numerator and the denominator alike and no threshold can fall when it
+// breaks. `--require-source-files` would have caught this under src/ or
+// scripts/, but the loader lives under tests/tools/.
+//
+// Three behaviours are worth pinning, and each is invisible in a different way:
+//
+//   * The loader must overlay against `this.resourcePath`. Against any other
+//     path no overlay matches and the data file passes through unchanged --
+//     the build then succeeds, serves the checked-in corpus, and the specs
+//     that exist to drive the overlaid branches quietly assert against the
+//     wrong document.
+//   * It must call `addDependency` for every overlay it applied. Dropping that
+//     breaks nothing in a clean CI build and breaks incremental rebuilds
+//     locally: editing a fixture leaves the previous overlay compiled in,
+//     which is the failure mode most likely to be read as a flaky spec.
+//   * A data file with no committed overlay must come back byte-for-byte. The
+//     overlay engine round-trips JSON through JSON.stringify, so a loader that
+//     forwarded unoverlaid files through it would silently reformat every
+//     other data file in the build.
+//
+// The cases below drive the real committed overlays rather than fixtures of
+// their own: the loader's contract is with those files, and a test that
+// supplied its own would not notice the wiring changing underneath it.
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { readFileSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import { join } from 'node:path';
+
+const require = createRequire(import.meta.url);
+const loader = require('./tools/e2e-data-fixture-loader.cjs');
+const {
+  DATA_DIR,
+  FIXTURE_DIR,
+  VARIANT_FIXTURE_DIR,
+  overlayPathsFor,
+} = require('./tools/e2e-data-fixtures.cjs');
+
+// A minimal stand-in for the webpack loader context: the loader reads
+// `this.resourcePath` and calls `this.addDependency`, and nothing else.
+function loaderContext(resourcePath) {
+  const dependencies = [];
+  return {
+    resourcePath,
+    addDependency(path) {
+      dependencies.push(path);
+    },
+    dependencies,
+  };
+}
+
+function run(relativeDataPath, { variant = false } = {}) {
+  const resourcePath = join(DATA_DIR, relativeDataPath);
+  const source = readFileSync(resourcePath, 'utf8');
+  const context = loaderContext(resourcePath);
+  // The loader reads the variant flag through process.env, so setting it here
+  // is the only way to drive the second pass. A unit run never arrives with it
+  // set -- the variant build is a separate pass of `npm run
+  // build:e2e:coverage` -- so the helper asserts that and restores by
+  // deleting, rather than carrying a restore branch no test can reach.
+  assert.equal(
+    process.env.E2E_COVERAGE_VARIANT,
+    undefined,
+    'E2E_COVERAGE_VARIANT leaked into the unit run',
+  );
+  if (variant) process.env.E2E_COVERAGE_VARIANT = '1';
+  try {
+    return { source, context, patched: loader.call(context, source) };
+  } finally {
+    delete process.env.E2E_COVERAGE_VARIANT;
+  }
+}
+
+test('the loader overlays the file named by this.resourcePath', () => {
+  const { source, patched } = run('members.json');
+
+  // tests/e2e/fixtures/data/members.json appends one organization holding both
+  // membership roles -- a status no real member carries. Its presence is proof
+  // the overlay for *this* path was found and applied; an overlay keyed off
+  // any other path would leave the corpus as committed.
+  const before = JSON.parse(source).members;
+  const after = JSON.parse(patched).members;
+  assert.equal(after.length, before.length + 1);
+  assert.equal(
+    before.some((member) => member.id === 'coverage-fixture-org'),
+    false,
+  );
+  assert.equal(
+    after.some((member) => member.id === 'coverage-fixture-org'),
+    true,
+  );
+});
+
+test('the loader registers every overlay it applied as a dependency', () => {
+  const { context } = run('members.json');
+
+  assert.deepEqual(context.dependencies, [join(FIXTURE_DIR, 'members.json')]);
+});
+
+test('the variant build layers both overlays and registers both', () => {
+  const resourcePath = join(DATA_DIR, 'community-people.json');
+  // community-people.json is the one data file carrying an overlay in each
+  // directory, so it is the only path on which the ordering is observable.
+  assert.deepEqual(
+    overlayPathsFor(resourcePath, { E2E_COVERAGE_VARIANT: '1' }),
+    [
+      join(FIXTURE_DIR, 'community-people.json'),
+      join(VARIANT_FIXTURE_DIR, 'community-people.json'),
+    ],
+  );
+
+  const { patched, context } = run('community-people.json', { variant: true });
+
+  assert.deepEqual(context.dependencies, [
+    join(FIXTURE_DIR, 'community-people.json'),
+    join(VARIANT_FIXTURE_DIR, 'community-people.json'),
+  ]);
+  // The variant overlay empties fetchedAt and is applied second, so seeing it
+  // win proves the base overlay did not overwrite it on the way past.
+  assert.equal(JSON.parse(patched).fetchedAt, '');
+});
+
+test('a data file with no committed overlay is returned byte-for-byte', () => {
+  // Chosen from the data directory at run time rather than named here: which
+  // files carry an overlay changes as fixtures are added, and this case is
+  // about the absence of one, not about a particular document.
+  const unoverlaid = [
+    'projects-born.json',
+    'milestones.json',
+    'metrics.json',
+    'awards.json',
+  ].find((name) => overlayPathsFor(join(DATA_DIR, name), {}).length === 0);
+  assert.ok(unoverlaid, 'every candidate data file now carries a base overlay');
+
+  const { source, patched, context } = run(unoverlaid);
+
+  assert.equal(patched, source);
+  assert.deepEqual(context.dependencies, []);
+});