diff --git a/tests/coverage-gate-thresholds.test.mjs b/tests/coverage-gate-thresholds.test.mjs
index 4ae2427b..6a427887 100644
--- a/tests/coverage-gate-thresholds.test.mjs
+++ b/tests/coverage-gate-thresholds.test.mjs
@@ -48,15 +48,16 @@ const FLOORS = {
   // entered none of the ratios above, which are all built from
   // isSourceFile(). These are the same three shapes over its own row.
   // Measured on the suite this commit ships: harness files 100.00% lines /
-  // 99.32% regions (3890/3890 lines, 1471/1481 regions), and the lowest
-  // harness file is tests/tools/e2e-coverage-report.mjs at 97.98%, floored
+  // 99.53% regions (3925/3925 lines, 1467/1474 regions), and the lowest
+  // harness file is tests/tools/e2e-coverage-report.mjs at 98.57%, floored
   // to 97. Every other harness file is at 100.00%.
   //
-  // 97.98% is also that file's ceiling, not a backlog: its 10 uncovered
+  // 98.57% is also that file's ceiling, not a backlog: its 7 uncovered
   // regions are the defensive fallbacks the note at the top of the file lists
-  // as unreachable (#1253), so no test can lift it. The per-file floor must
-  // stay at or below 97 until that list shrinks; a probe that lands one of
-  // them should raise the floor and shorten the list in the same change.
+  // as unreachable (#1253; three more were deleted rather than listed in
+  // #1304), so no test can lift it. The per-file floor must stay at or below
+  // 98 until that list shrinks; a probe that lands one of them should raise
+  // the floor and shorten the list in the same change.
   '--check-harness': 100,
   '--check-harness-regions': 97,
   '--check-harness-file-regions': 97,
diff --git a/tests/e2e-coverage-attribution.test.mjs b/tests/e2e-coverage-attribution.test.mjs
new file mode 100644
index 00000000..44852585
--- /dev/null
+++ b/tests/e2e-coverage-attribution.test.mjs
@@ -0,0 +1,238 @@
+// `convertScript` (tests/tools/e2e-coverage-report.mjs) decides which of a
+// bundle's original sources a script's coverage may be written to:
+//
+//   const attributedPaths = new Set(
+//     Object.entries(converter.branches)
+//       .filter(([, ranges]) => ranges.length > 0)
+//       .map(([path]) => path),
+//   );
+//
+// and `collectE2ECoverage` drops any `src/` path missing from that set when
+// the bundle maps to more than one source. So "branches alone decide
+// attribution" is a load-bearing claim about v8-to-istanbul, not a style
+// choice: a release that stopped populating `branches` for a source would not
+// fail anything -- the report would quietly stop counting that file, and the
+// e2e gate would pass on a smaller denominator.
+//
+// These tests state the facts the single-arm form rests on. They drive the
+// installed converter directly rather than a report, because a report can only
+// show the consequence and these are the premises:
+//
+//   1. `branches` and `functions` are initialised to `{}` by the converter's
+//      constructor, so neither needs a `?? {}` guard;
+//   2. `functions` is populated only for a *named* block, and then only for
+//      the source its first range lands in, which makes it a strictly weaker
+//      signal than `branches`; and
+//   3. the records `convertScript` applies are always anonymous, because
+//      `normalizeRanges` rebuilds every script's coverage as one block with
+//      `functionName: ''` -- which is why a `functions` arm could never have
+//      contributed a path that `branches` had not.
+//
+// The last test is the end-to-end consequence, kept beside the premises: a
+// two-source bundle attributes both of its sources, including one whose
+// coverage is a single uninterrupted covered range.
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { mkdir, mkdtemp, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { dirname, join } from 'node:path';
+
+import v8ToIstanbul from 'v8-to-istanbul';
+
+import { collectE2ECoverage } from './tools/e2e-coverage-report.mjs';
+import {
+  initCoverageRun,
+  sealCoverageRun,
+  writeCoverageArtifact,
+} from './tools/e2e-coverage-run.mjs';
+
+const ALPHA = 'const alpha1 = 1;\nconst alpha2 = 2;\n';
+const BETA = 'const beta1 = 1;\nconst beta2 = 2;\n';
+const MAPPED_SOURCE = `${ALPHA}${BETA}`;
+const SCRIPT_TEXT = `${MAPPED_SOURCE}\n//# sourceMappingURL=app.js.map\n`;
+const ALPHA_PATH = 'src/components/Alpha/index.js';
+const BETA_PATH = 'src/components/Beta/index.js';
+
+// Four generated lines, all at column 0. 'AAAA' opens source 0 line 1, 'AACA'
+// advances the original line, 'ACDA' switches to source 1 and steps the
+// original line back to 1 (source +1, originalLine -1), 'AACA' advances it
+// again. Spelled out rather than produced by a helper so the fixture states
+// exactly where the source boundary falls.
+const MAPPINGS = 'AAAA;AACA;ACDA;AACA';
+
+// `normalizeSourceMap` rewrites every reference to the resolved absolute path
+// of the file on disk before the converter sees it, so the premise tests use
+// that shape rather than the `webpack://` form the bundler emits.
+const ABSOLUTE_SOURCES = ['/virtual/src/alpha.js', '/virtual/src/beta.js'];
+
+function premiseMap() {
+  return {
+    version: 3,
+    file: 'app.js',
+    sourceRoot: '',
+    sources: [...ABSOLUTE_SOURCES],
+    sourcesContent: [ALPHA, BETA],
+    names: [],
+    mappings: MAPPINGS,
+  };
+}
+
+// One range per source, which is what `splitRangeAtBoundaries` guarantees
+// `convertScript` hands to `applyCoverage`: a range that straddles the
+// boundary resolves its endpoints into two different originals, and
+// v8-to-istanbul abandons it, recording nothing for either source.
+function blockCovering(functionName) {
+  return [
+    {
+      functionName,
+      isBlockCoverage: true,
+      ranges: [
+        { startOffset: 0, endOffset: ALPHA.length - 1, count: 1 },
+        {
+          startOffset: ALPHA.length,
+          endOffset: MAPPED_SOURCE.length,
+          count: 1,
+        },
+      ],
+    },
+  ];
+}
+
+function newConverter() {
+  return v8ToIstanbul('/virtual/build/assets/js/app.js', 0, {
+    source: MAPPED_SOURCE,
+    sourceMap: { sourcemap: premiseMap() },
+  });
+}
+
+async function applied(functionName) {
+  const converter = newConverter();
+  await converter.load();
+  converter.applyCoverage(blockCovering(functionName));
+  return converter;
+}
+
+// The same fold `convertScript` applies, so these tests read the maps exactly
+// as attribution does rather than merely asserting they are non-empty.
+const attributable = (map) =>
+  Object.entries(map)
+    .filter(([, ranges]) => ranges.length > 0)
+    .map(([path]) => path)
+    .sort();
+
+test('the converter initialises branches and functions, so neither needs a ?? guard', async () => {
+  const converter = newConverter();
+  await converter.load();
+  assert.deepEqual(converter.branches, {});
+  assert.deepEqual(converter.functions, {});
+
+  // Still object-valued once coverage has been applied. The `?? {}` guards
+  // `convertScript` used to carry could only ever have fired if one of these
+  // were reassigned to a nullish value, which nothing in the converter does.
+  converter.applyCoverage(blockCovering(''));
+  assert.equal(typeof converter.branches, 'object');
+  assert.notEqual(converter.branches, null);
+  assert.equal(typeof converter.functions, 'object');
+  assert.notEqual(converter.functions, null);
+});
+
+test('an anonymous block attributes every source through branches and none through functions', async () => {
+  const converter = await applied('');
+  assert.deepEqual(attributable(converter.branches), ABSOLUTE_SOURCES);
+  // The whole reason the `functions` arm of `attributedPaths` was removable:
+  // `normalizeRanges` names no block, so this map is never written to.
+  assert.deepEqual(converter.functions, {});
+});
+
+test('a named block populates functions, and then only for its first range', async () => {
+  // Non-vacuity for the test above -- `functions` is not simply always empty --
+  // and the reason it would remain the weaker arm even if it were reachable: a
+  // named block records one function, against the source its first range lands
+  // in, while `branches` already names both.
+  const converter = await applied('render');
+  assert.deepEqual(attributable(converter.functions), [ABSOLUTE_SOURCES[0]]);
+  assert.deepEqual(attributable(converter.branches), ABSOLUTE_SOURCES);
+});
+
+async function fixtureRun() {
+  const root = await mkdtemp(join(tmpdir(), 'endusers-attribution-'));
+  const runDir = join(root, 'coverage');
+  const buildDir = join(root, 'build');
+  for (const [path, content] of [
+    [ALPHA_PATH, ALPHA],
+    [BETA_PATH, BETA],
+  ]) {
+    await mkdir(dirname(join(root, path)), { recursive: true });
+    await writeFile(join(root, path), content);
+  }
+  await mkdir(join(buildDir, 'assets/js'), { recursive: true });
+  await writeFile(join(buildDir, 'assets/js/app.js'), SCRIPT_TEXT);
+  await writeFile(
+    join(buildDir, 'assets/js/app.js.map'),
+    JSON.stringify({
+      version: 3,
+      file: 'app.js',
+      sources: [
+        `webpack://endusers/./${ALPHA_PATH}`,
+        `webpack://endusers/./${BETA_PATH}`,
+      ],
+      sourcesContent: [ALPHA, BETA],
+      names: [],
+      mappings: MAPPINGS,
+    }),
+  );
+  await initCoverageRun(runDir, 'run-1', { project: 'chromium' });
+
+  // A zero-count range inside Alpha's line 2, which is what the browser
+  // records for a block that did not run. Beta is left as one uninterrupted
+  // covered range, so it is the source that tests whether attribution
+  // survives a file the run never split.
+  const alphaSplitStart = ALPHA.indexOf('const alpha2');
+  await writeCoverageArtifact(runDir, 'worker-0-page-0', {
+    schemaVersion: 1,
+    kind: 'endusers.playwright.v8-coverage',
+    runId: 'run-1',
+    result: [
+      {
+        url: 'http://localhost:3000/assets/js/app.js',
+        scriptId: '1',
+        functions: [
+          {
+            functionName: '',
+            isBlockCoverage: true,
+            ranges: [
+              { startOffset: 0, endOffset: SCRIPT_TEXT.length, count: 1 },
+              {
+                startOffset: alphaSplitStart,
+                endOffset: alphaSplitStart + 'const alpha2'.length,
+                count: 0,
+              },
+            ],
+          },
+        ],
+      },
+    ],
+  });
+  await sealCoverageRun(runDir, 'passed');
+  return { root, runDir, buildDir };
+}
+
+test('both sources of a two-source bundle are attributed, including the unsplit one', async () => {
+  const fixture = await fixtureRun();
+  try {
+    const report = await collectE2ECoverage(fixture.runDir, {
+      root: fixture.root,
+      buildDir: fixture.buildDir,
+    });
+    assert.equal(report.status, 'ok');
+    // `sourceFiles.length > 1` here, so a path missing from `attributedPaths`
+    // is dropped from the report outright -- silently, with no diagnostic.
+    assert.deepEqual(
+      report.sources.map((source) => source.file).sort(),
+      [ALPHA_PATH, BETA_PATH].sort(),
+    );
+    assert.deepEqual(report.diagnostics.errors, []);
+  } finally {
+    await rm(fixture.root, { recursive: true, force: true });
+  }
+});
diff --git a/tests/tools/e2e-coverage-report.mjs b/tests/tools/e2e-coverage-report.mjs
index 014569d4..a5e2c75e 100644
--- a/tests/tools/e2e-coverage-report.mjs
+++ b/tests/tools/e2e-coverage-report.mjs
@@ -45,10 +45,6 @@ const ORIGINAL_SCRIPT = /\.(?:c|m)?(?:js|jsx|ts|tsx)$/u;
 //       `c` satisfies `c < range.endOffset` by the filter above it, `start`
 //       equals `c` when the loop ends, so the final `parts.push` always runs
 //       and `parts` is never empty.
-//   convertScript       `converter.branches ?? {}` and `converter.functions
-//       ?? {}`, and the `.map(([path]) => path)` arm that only runs when one
-//       of those is absent -- `v8-to-istanbul` always defines both after
-//       `load()`, so the fallback needs a shape the library does not emit.
 //   collectE2ECoverage  `converted.coverage ?? {}` -- `convertScript` returns
 //       `converter.toIstanbul()` on every path and that never returns nullish;
 //       and `if (lineCoverage.size === 0) continue`, which needs an istanbul
@@ -59,7 +55,10 @@ const ORIGINAL_SCRIPT = /\.(?:c|m)?(?:js|jsx|ts|tsx)$/u;
 //
 // Reachability was established empirically rather than by reading: probe tests
 // were written against each candidate and these are the ones no probe could
-// close. Together they are 10 regions, which caps this file at 97.98% regions;
+// close. (The `convertScript` guards that used to sit in this list were
+// removed outright once tests/e2e-coverage-attribution.test.mjs pinned the
+// converter shape they defended against, #1304.) Together they are 7 regions,
+// which caps this file at 98.57% regions;
 // tests/coverage-gate-thresholds.test.mjs records that ceiling beside
 // `--check-harness-file-regions` so the floor is never set above it. Raise the
 // floor rather than this list when a probe does land, and extend this list
@@ -682,14 +681,28 @@ async function convertScript(scriptCoverage, root, scriptRoots) {
   }
   converter.applyCoverage(usableFunctions);
   const coverage = converter.toIstanbul();
-  const attributedPaths = new Set([
-    ...Object.entries(converter.branches ?? {})
+  // `converter.branches` is the whole attribution signal, and the two things
+  // this does not also read are not an oversight:
+  //
+  //   * `converter.functions` is permanently `{}` here. v8-to-istanbul writes
+  //     `this.functions[path]` only under `if (block.functionName ...)`
+  //     (lib/v8-to-istanbul.js:167,178), and `normalizeRanges` above rewrites
+  //     every script's record into exactly one block with `functionName: ''`.
+  //     A `functions` arm would therefore contribute no path that `branches`
+  //     has not already contributed.
+  //   * neither needs a `?? {}` guard: both are initialised to `{}` in the
+  //     converter's constructor (lib/v8-to-istanbul.js:36-37) and are never
+  //     reassigned.
+  //
+  // tests/e2e-coverage-attribution.test.mjs pins both facts against the
+  // installed converter, so a release that started naming blocks or stopped
+  // initialising the maps fails there rather than silently dropping a source
+  // from a multi-source bundle.
+  const attributedPaths = new Set(
+    Object.entries(converter.branches)
       .filter(([, ranges]) => ranges.length > 0)
       .map(([path]) => path),
-    ...Object.entries(converter.functions ?? {})
-      .filter(([, ranges]) => ranges.length > 0)
-      .map(([path]) => path),
-  ]);
+  );
   return {
     sourceFiles,
     coverage,