[Bug]: Unicode regex with a one-character class before $ misses letters outside the BMP on Electron 44 (composerContextLegacy test fails) #18272

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

packages/contracts or packages/shared

Steps to reproduce

1. On main, run vp test run src/composerContextLegacy.test.ts from packages/shared.
2. The case does not replace terminal labels embedded in 𐐀@build:7 fails.

Minimal engine repro (no project code), saved as probe.js:

const s = "\u{10400}"; // DESERET CAPITAL LETTER LONG I, a letter outside the BMP
console.log(process.versions.v8, {
  classBeforeEnd: /[\p{L}]$/u.test(s),         // false (wrong)
  literalBeforeEnd: /[\u{10400}]$/u.test(s),   // false (wrong)
  alternation: /(?:\p{L})$/u.test(s),          // true
  quantified: /[\p{L}]+$/u.test(s),            // true
  unanchored: /[\p{L}]/u.test(s),              // true
});

Run it with ELECTRON_RUN_AS_NODE=1 <the desktop app's electron binary> probe.js, or with Node 26.5.

Expected behavior

/[\p{L}\p{N}\p{M}_@.-]$/u (packages/shared/src/composerContextLegacy.ts, line 423) matches when the text before the label ends in a letter outside the BMP. The legacy-context upgrade then leaves a terminal label glued to that letter as plain text, as the test expects.

Actual behavior

On the affected engines, a single character class directly before $ with the u flag never matches a character outside the BMP, even when the class names that exact code point. So line 423 returns false and the label is turned into a chip.

It is an engine fault, not a logic error: the same check written as an alternation is correct on the same engines. Checks anchored at the start (^[x]), quantified classes, and unanchored classes read correctly.

Impact

Minor bug or occasional failure

Version or commit

main @ 2625327

Environment

macOS 27.0.1. Fails on Electron 44.4.5 (V8 15.2.124.28-electron.0, the version in pnpm-workspace.yaml) and on Node 26.5.0 (reports V8 14.6.202.34-node.24), which the test runner uses here.

Workaround

Use an alternation instead of a one-character class before $. This passes the failing test and matches the astral letter on both engines above:

/(?:\p{L}|\p{N}|\p{M}|[_@.-])$/u.test(body.slice(0, at))

A search of apps/ and packages/ found no other unanchored one-character class before $ that can meet a character outside the BMP. The engine fault itself probably belongs upstream in V8.