[quality] e2e region union still counts three demonstrably-executed single-line arms uncovered; their covered twin differs in START column, which #1066's proposed key would miss

Start with the union in tests/tools/e2e-coverage-report.mjs and its tests in tests/e2e-coverage-report.test.mjs; coordinate with PR #1051, which is rewriting both files. Add a regression test for a differing-start-column pair, while checking that the listed MemberDirectory/index.js regions remain distinct. Re-derive --check-source-regions afterward; that step touches .github/workflows/ci.yml and requires Workflows permission.

Finding

Three single-line regions that the e2e suite demonstrably executes are still
counted as uncovered, and neither of the two fixes currently in flight folds
them.

This is not a restatement of #1035
or #1066; it is the residue both
leave behind, with three cases whose execution is provable from a passing
assertion rather than inferred.

The three regions

All three are single-line, zero-count, and sit in code a spec that passed in
the same run had to execute:

Why #1051 does not fold them

#1051's isPhantomRegion returns
early on region.endLine <= region.line, so it only ever drops multi-line
zero regions. All three regions above are single-line. Re-rendering this run
with #1051 applied leaves all three exactly as they were:

src/components/MemberDirectory/MemberCard.js    | 100.00 | 96.15 |  | 75
src/components/MemberDirectory/MemberProfile.js | 100.00 | 92.31 |  | 10 15

#1051 is still correct and still worth landing — it lifts src files from
80.90% to 87.88% on this run by folding the multi-line cases, including
the whole-function span described below. This issue is only about what survives
it.

Why #1066's proposed key does not fold them either

#1066 observes drifted halves
with the same start column and a differing end column, and proposes keying on
startLine:startColumn plus the branch's ordinal within the line.

For these three, the start column differs too, so that key keeps them apart:

MemberProfile.js   zero 10:40:10:48    covered twin 10:44:15:19
MemberProfile.js   zero 15:19:15:80    covered twin 15:70:38:15
MemberCard.js      zero 75:19:75:40    covered twin 75:33:81:1

Dumped straight from the union site in tests/tools/e2e-coverage-report.mjs,
instrumented to print each region key with its script URL and artifact. Both
halves of each pair come from the same chunk (c1ce2b9c.8f0a7cfe.js), so
the real-build/variant-build explanation in #1066 does not apply here — the
drift is between two artifacts of one chunk.

A worked example, the two artifacts of tests/e2e/member-directory.spec.js:

<artifact A>  [('15:19:15:80', 0), ('15:70:35:24', 1), ('35:24:38:15', 0), ('38:14:49:51', 1)]
<artifact B>  [('10:40:10:48', 0), ('10:44:15:19', 1), ('15:19:15:80', 0), ('15:70:38:15', 1), ('38:14:43:11', 0)]

Note 15:70:35:24 in A against 15:70:38:15 in B — the same source branch,
same start, two different ends, from one chunk. That pair is the #1066 shape and
an ordinal key would fold it. The 10:40 / 10:44 pair on the next line is the
shape that key misses.

A second, separable mechanism in the same union

While isolating the above I found why a never-executed function inflates the
denominator. Of the 17 artifacts that carry MemberProfile.js:

regions=  1  has_whole_fn_range=True   covered= 0  -> 10 artifacts
regions= 11  has_whole_fn_range=False  covered= 9  ->  1 artifact
regions= 13  has_whole_fn_range=False  covered=10  ->  3 artifacts
regions= 15  has_whole_fn_range=False  covered= 8  ->  1 artifact
regions= 20  has_whole_fn_range=False  covered=11  ->  2 artifacts

Ten artifacts — pages that load the member-directory chunk without ever opening
a dialog — contribute exactly one region, 8:7:170:1, the whole
MemberProfile function, count 0. V8 emits one coarse range for a function that
never runs and fine-grained block ranges once it does. Because the union keys on
exact coordinates, that coarse range can never be cancelled by the fine ranges,
and it enters the denominator permanently.

#1051 already folds this one (it is multi-line over fully covered lines). It is
recorded here because it is the reason #1051's heuristic works, and an exact
fix should handle it deliberately rather than as a side effect.

Recommendation

Make region identity survive the same source branch being mapped to different
spans in different artifacts of one chunk, covering the differing-start-column
case, not only the differing-end-column case:

1. In getRegionCoverage() / the union in tests/tools/e2e-coverage-report.mjs,
   fold a zero-count region into a covered one when the covered region's span
   contains it and both derive from the same branch. Containment folds all
   three pairs above, and the 8:7:170:1 whole-function range, without the
   start-column assumption in #1066.
2. Keep single-line regions that have no containing covered twin distinct.
   Several genuinely independent arms share a line — MemberDirectory/index.js
   lines 31/34/40/43, which #1060
   is covering for real — and collapsing those would hide real gaps. Whatever
   rule lands must leave those four still uncovered until #1060 merges.
3. Add a regression test to tests/e2e-coverage-report.test.mjs driving the
   union with two artifacts of one script whose maps give one branch a
   differing start column, asserting it folds to a single region counted
   covered.
4. Re-derive --check-source-regions afterwards.

Coordination

• The fix lands in tests/tools/e2e-coverage-report.mjs and
  tests/e2e-coverage-report.test.mjs, both of which open PR
  #1051 is currently rewriting.
  I have deliberately not opened a PR: anything I pushed would be a second
  implementation over the same two files. This should be built on top of #1051
  once it lands, not instead of it.
• Related but distinct: #1035 (exercised branches reported uncovered — the
  multi-line instances of which #1051 fixes) and #1066 (real-build vs
  variant-build drift, differing end columns).
• Step 4 touches .github/workflows/ci.yml. That path cannot be pushed by this
  lane's token — GitHub rejects a workflow diff from a contributor-tier app
  token — so re-deriving the floor needs a human or an agent holding the
  Workflows permission. That is a hard ceiling, not a preference.

Evidence and provenance

• Unit: npm run test:unit:coverage (TZ=UTC node tests/tools/coverage-report.mjs,
  node v26.10.0), run locally at 900592b. src files 100.00% lines /
  99.84% regions; the only uncovered regions in the whole tree are four known
  unreachable ?? '' fallbacks in scripts/lib/svg-active-content.mjs. These
  are therefore e2e-report defects, not source gaps.
• E2E: local npm run build:e2e:coverage, then
  node tests/tools/e2e-coverage-run.mjs init --dir <dir> --run-id <id>,
  npm run test:e2e:coverage with E2E_COVERAGE_DIR/E2E_COVERAGE_RUN_ID set
  (298 passed, 0 failed), then seal, then
  node tests/tools/e2e-coverage-report.mjs --input <dir> --build build.
  Revision 900592b. Result src files 466 regions / 377 covered / 80.90%,
  which reproduces CI job End-to-end coverage (workflow Validate repository,
  run 37164361552)
  at 468 / 379 / 80.98% to within one region.
• Region keys dumped by instrumenting the union in
  tests/tools/e2e-coverage-report.mjs to print {file, key, count, scriptUrl, artifact}. The instrumentation was local only and is not proposed for commit.
• The #1051 numbers come from re-rendering the same artifacts with
  tests/tools/e2e-coverage-report.mjs taken from pull/1051/head.

Completion criteria

• A zero-count region contained by a covered region from another artifact of the same chunk folds into it, including when the start columns differ
• MemberProfile.js 10 and 15 and MemberCard.js 75 are no longer reported uncovered, with no new test written for them
• MemberDirectory/index.js lines 31, 34, 40 and 43 remain reported uncovered until #1060 merges
• A regression test in tests/e2e-coverage-report.test.mjs covers the differing-start-column union
• --check-source-regions re-derived from the corrected number

Priority

• Impact: medium (the gate measures regions that are already exercised, and the fix proposed in #1066 will not reach these)
• Effort: medium

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