Node 21+ has a getter-only navigator global, so assigning a test stub throws in ES modules and is ignored in scripts

Fact

Since Node 21, globalThis.navigator exists (userAgent is "Node.js/<major>") and is an accessor with a getter and no setter. globalThis.navigator = stub throws TypeError: Cannot set property navigator of #<Object> which has only a getter in an ES module, and in a sloppy CommonJS script it silently does nothing, leaving the real object in place.

Why it matters

Unit tests that run browser code in Node by stubbing navigator (iOS standalone, userAgent, clipboard, mediaDevices) break on upgrade: an ESM suite dies at the assignment, and a CommonJS one keeps running against Node's own navigator, so properties the stub set read undefined and the code under test silently takes the "not a browser" path. Feature checks like typeof navigator !== 'undefined' also start reporting Node as a browser.

How to apply

Install and restore the stub through property descriptors:

const real = Object.getOwnPropertyDescriptor(globalThis, 'navigator');
Object.defineProperty(globalThis, 'navigator', { value: { standalone: false }, configurable: true });
// ...test...
Object.defineProperty(globalThis, 'navigator', real);

Or run the suite with --no-experimental-global-navigator. Detect a browser by typeof document, not by navigator.