PTY snapshot suite: rare non-repeating case failures under parallel load (pass in isolation)

[simulacre7]

Summary

Running the full migration* PTY snapshot suite in parallel occasionally fails a small, non-repeating set of cases (~0.4% of case-executions). Every affected case passes deterministically when run in isolation, and the failing names differ between runs — so this looks like load-sensitivity in the PTY runner environment rather than any individual fixture or product bug. Filing per m0g3r's suggestion in #2483 after it showed up there first.

Observations (macOS arm64, M-series, just snapshot-test migration)

Five full-suite runs across two commits, all with a fresh packages/cli/dist (freshness guard green):

run	commit	result	failed cases
A	31163c5 (PR #2483 branch)	151/154	migration_dynamic_oxc_configs, migration_framework_shim_vue, migration_standalone_yarn4_idempotent
B	cc20535d (main)	159/161	migration_not_supported_vitest3, migration_from_tsup_monorepo_success
C	cc20535d (main)	161/161	—
D	cc20535d (main)	161/161	—
E	31163c5, earlier same-day	148/154	different set again (incl. migration_husky_or_prepare, migration_standalone_bun_install)

• No case name repeats across runs. Union of failures over five runs: 10+ distinct fixtures, each failing exactly once.
• Every one of them passes in isolation (just snapshot-test <name>), immediately after the failing run, unchanged tree.
• The failure-heavy runs were the first suite run after a fresh build / cold caches; warm re-runs (C, D) were fully green. Cold-state work on first touch (managed runtime provisioning, registry-bridge first hits) overlapping with ~14 parallel PTY cases is my best guess at the mechanism, but I have not isolated it.
• m0g3r additionally verified in feat(migrate): preserve dynamic Oxlint and Oxfmt configs #2483 that the flaked fixtures are structurally unremarkable (step counts mid-pack; one of 21 double-vp migrate fixtures).

Why it may matter

Locally it's a shrug; in CI a 0.4% per-case flake across ~650 cases makes a meaningful fraction of runs red for reasons unrelated to the diff under test, and the changing names make it hard for contributors to tell signal from noise (it cost a round of triage on #2483).

Environment

• macOS arm64 (Darwin 25.2), 14 threads reported by the runner
• just snapshot-test migration (both flavors), toolchain per rust-toolchain.toml

Happy to re-run with any added instrumentation (timings, per-case retries, runner verbosity) if that helps narrow it — I can reproduce roughly one flaky run per two or three cold full-suite runs.