[JackyHsu-X]

Fixes #1582.

Prepare the main thread's event loop before starting the kernel. Handle Python 3.12's implicit-loop deprecation by explicitly creating a loop, while preserving an existing loop. Make IPythonKernel.do_shutdown() async, matching the base class and avoiding its synchronous-hook warning.

Seven regression cases cover unset, cleared and preconfigured loops, both shutdown restart flags, and real stock-launcher startup, execution, shutdown replies and clean process exits with warnings-as-errors. The subprocess cases explicitly opt into JUPYTER_PLATFORM_DIRS=1 so jupyter_core 5.1.0's unrelated path-migration warning does not terminate imports. Their strict warning policy remains enabled.

Validation:

• Linux/Python 3.11.18 with all 54 package versions from the failed minimum-dependency CI job pinned: published tests reproduce its five failures; the revised cases plus direct shutdown tests pass all seven. The remote interpreter was 3.11.17, so this is not an identical runner image.
• Original production with the revised tests still fails both lifecycle shutdown checks and both direct async-shutdown cases. Original production on Windows/Python 3.12.8 still fails the unset-loop case; configured and cleared loops pass.
• Complete local Linux minimum suite: 236 passed, 18 skipped, one headless TkAgg failure reproduced on unchanged production with the same dependencies; 13 existing warnings retained. This is not a full-suite pass.
• Existing Windows/Python 3.12.8: all five affected cases pass after the follow-up. The previous full-suite result on the initial patch was 180 passed, 32 skipped; it was not rerun for this test-only revision. Previous Linux/Python 3.12 results and typing baseline limits remain documented in the earlier validation.
• Applicable official pre-commit hooks and diff checks pass for the revised tests. No new production typing result is claimed; the earlier official Windows mypy run had the same 15 baseline diagnostics on both production versions.

No production code changed in the minimum-dependency follow-up. No dependency, lockfile or CI changes. The full platform/Python/GUI matrix was not run locally, and remote CI must run on the revised head. The separate triage-label gate needs a maintainer.

AI disclosure: OpenAI Codex generated the changes and ran the investigation and validation. No independent human review is claimed.

[JackyHsu-X]

The minimum-dependency failure occurs before the new cases reach the kernel lifecycle checks: jupyter_core 5.1.0 emits its platformdirs migration warning during import, and the child interpreters deliberately use warnings-as-errors.

The follow-up opts these two test functions into the documented JUPYTER_PLATFORM_DIRS=1 behavior using monkeypatch. PYTHONWARNINGS=error and -Werror remain enabled. Production code, warning filters, dependencies and CI are unchanged.

Validation:

• Linux Python 3.11.18 with all 54 package versions from the failed job pinned: the published tests reproduce all five failures; the revised cases plus direct shutdown tests pass all seven.
• Original production with these revised tests still fails both real shutdown lifecycles and both direct async-shutdown cases. On Windows/Python 3.12.8, its unset-loop case still fails while the two loop controls pass. The Linux reproduction uses Python 3.11.18 rather than the job's 3.11.17.
• Complete local minimum suite: 236 passed, 18 skipped, one headless TkAgg failure. That same Tk failure also occurs on original production with the same dependencies; this is not a full-suite pass. The five affected Windows cases and applicable pre-commit hooks pass.

The remaining label gate requires a triage label such as bug from a maintainer. Remote CI will still need to run on the follow-up commit.

OpenAI Codex prepared and ran this investigation, revision and validation; no independent human review is claimed.