Default CustomEvent detail to null when omitted

[olavoasantos]

Problem

CustomEvent.detail was assigned directly from the optional initialization dictionary. Constructing a CustomEvent without a detail property therefore exposed undefined, whereas the event’s default detail value is null.

Impact

Minor. Consumers that distinguish an omitted payload from an explicitly supplied value can observe the wrong public event contract. In the worker polyfill, that makes CustomEvent behave differently from the DOM API it models.

Reproduction

new CustomEvent('change').detail;
new CustomEvent('change', {}).detail;

Before this change, both expressions evaluated to undefined. They now evaluate to null. The regression test also shows that explicitly supplied null, 0, false, '', and object values are preserved; an explicitly undefined detail is normalized to null by this implementation.

Change

Normalize the constructor’s detail value to null when it is nullish. The same normalization is applied by the legacy initCustomEvent() path so both ways of creating a custom event expose the same default.

Tests

Adds focused constructor coverage for an omitted initialization dictionary, a dictionary without detail, explicit undefined, and explicitly supplied falsy and object values.

Stack

• Stack: Independent quick wins (#702), layer 2 of 4
• Base: fix-node-contains
• Review after PR Fix Node.contains ancestor traversal #688.

Validation

Fresh GitHub CI on restacked head 5f0b228 passes:

• lint;
• type-check and build;
• unit tests with coverage;
• bundle-size checks;
• Playwright end-to-end tests;
• the classified Web Platform Test suite.

[henrytao-me]

LGTM for the scoped default-detail fix, reviewed at 5f0b2281bddfc7ac37c64503f40f38f4f9b98243 against actual #688 base 3c57e5b6202f0779eb7ee99ad3298f6d17032c51.

The constructor and legacy initCustomEvent() now use the DOM null default for omitted/undefined detail while preserving explicit falsy values and payload identity. I found no introduced blocker.

Validation:

• 108 polyfill tests and 56 focused core tests pass.
• The eight added constructor cases produce 3 failures / 5 passes on the base, matching the intended default changes.
• All 43 isolated native/base/head comparisons match the head, including legacy defaults, payload identity, detail getters, original getter errors, and listener delivery.
• Locked Prettier passes; reported current-head CI is green.

Non-blocking test suggestion: add initCustomEvent() cases for resetting a populated event with omitted/undefined detail and preserving falsy/object values. This method changed too, while the new repository tests exercise only the constructor; the separate native probe verified both paths.

This approval is scoped code-review sign-off, not a full locked build or live Shell/admin tophat. Restack/recheck as the parent lands; integrated stack and live-validation gates remain separate.