Fix Node.contains ancestor traversal

[olavoasantos]

Problem

Node.contains() is supposed to walk from the candidate node toward the root until it finds the receiver. The previous loop repeatedly read the original candidate’s parent instead of advancing from the current node, so it stopped making progress after the first hop.

As a result, checks such as parent.contains(grandchild) and checks against a node in another attached subtree never returned.

Impact

Critical. This is a synchronous infinite loop. Because the polyfill is intended for non-browser environments such as Web Workers, one affected contains() call can block that worker’s event loop indefinitely instead of returning a boolean.

Reproduction

const parent = document.createElement('div');
const child = document.createElement('span');
const grandchild = document.createElement('em');

parent.appendChild(child);
child.appendChild(grandchild);

parent.contains(grandchild);

Before this change, the final call never returned. It now returns true. The equivalent check against a node in another subtree now terminates and returns false.

Change

Advance through currentNode.parentNode on each iteration and return false after reaching the root. This preserves the expected results for the receiver itself and direct children while allowing deeper and unrelated-node checks to terminate.

Tests

Regression coverage verifies the receiver itself, a direct child, a deep descendant, a node in another subtree, a detached node, and null. The deep-descendant and other-subtree cases exercise the paths that previously hung.

Stack

• Stack: Independent quick wins (#702), layer 1 of 4
• Base: main
• This is the bottom of its stack.

Coverage includes both the explicit detached-node case and a node attached to another subtree.

Validation

Fresh GitHub CI on restacked head 3c57e5b 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