WebApi: Navigator and Scheduler get a real interface object and prototype

[lahma]

Closes #3270. Navigator and Scheduler get the interface object and interface prototype object WebIDL gives them, the two singletons' members move onto those prototypes, and the three exemptions #3272 had to carve out disappear — WebIdlPropertyAttributeTests.Divergences is now [].

The shape, per interface

Both take the usual triple: interface object → [JsObject(UseShape = true)] : Prototype → a plain ObjectInstance singleton with no own properties at all.

• Navigator — NavigatorConstructor → NavigatorPrototype (constructor, @@toStringTag = "Navigator", userAgent as Configurable | Enumerable) → JsNavigator, which carries no state and exists to be the brand.
• Scheduler — SchedulerConstructor → SchedulerPrototype (constructor, @@toStringTag, postTask/yield as ConfigurableEnumerableWritable) → JsScheduler, which holds the SchedulerQueue and TimerQueue. That is the split WebIDL draws and the one JsPerformance already uses: the members are the interface's, the queues are the instance's, and each member brand-checks its receiver and then operates on it — so an extracted postTask behaves exactly as a browser's does.

Neither is constructible. HTML's [Exposed=Window] interface Navigator { } and the draft's [Exposed=(Window,Worker)] interface Scheduler { … } declare no constructor operation, so per WebIDL §3.7.1 each is a function that throws — TypeError: Illegal constructor, which is Node 24's exact message. TaskController and TaskPriorityChangeEvent in the same draft do declare one, which is why they stay constructible and this does not.

Why they are globals despite [Exposed=Window]. That question was settled when the singletons were installed: navigator carries the same [Exposed=Window] declaration and is a Jint global anyway, because WinterTC asks a non-browser runtime for globalThis.navigator.userAgent. Having decided the instance belongs, withholding the interface object leaves navigator instanceof Navigator unwritable while the object it names sits one link up the chain — the half-truth #3250 exists to remove. Node 24's global is not a Window either and exposes both. For Scheduler the draft status was already priced in: the feature installs three interface objects today, and withholding the fourth would make this the one API whose singleton cannot answer scheduler instanceof Scheduler while its own controller and signal can. The interface object has to exist regardless — Scheduler.prototype.constructor is it — so a global only settles reachability by name, and Chrome, the only implementation, exposes it.

Both installs follow #3250: lazy non-clobbering LazyPropertyDescriptor<Engine> on the principal realm's global, interface-object attributes {w:true, e:false, c:true}, never in a ShadowRealm, behind the singleton's own feature flag.

Against Node 24, run live: 16 of 18 identical

typeof Navigator, name/length, Reflect.ownKeys(navigator) → [], Object.keys(navigator) → [], both prototype links, instanceof, no own Symbol.hasInstance, constructor === Navigator, userAgent an accessor with get/no-set/enumerable:true/configurable:true on Navigator.prototype, new Navigator() → TypeError: Illegal constructor, constructor {w:t,e:f,c:t}, prototype {w:f,e:f,c:f}, containment.

Two differ, both deliberate:

	Jint	Node 24	why
Object.keys(Navigator.prototype)	["userAgent"]	adds hardwareConcurrency, locks, language, languages, platform	absent-rather-than-faked, unchanged policy — now re-pinned against the prototype rather than the instance
Object.prototype.toString.call(navigator)	[object Navigator]	[object Object]	Node's Navigator is an ordinary JS class with no @@toStringTag; WebIDL's class-string rule and a browser both say [object Navigator], and Jint's pre-existing answer is preserved by moving the tag to the prototype

Node has no scheduler at all, so there the oracle is the WICG IDL plus Chrome's shape.

The containment hazard #3266 identified is closed

Before, both singletons were BuiltinShapeObjects whose [[Prototype]] was Object.prototype, so patching "the scheduler's prototype" patched Object.prototype. Same script, before → after:

	before	after
({}).poisonedByNavigator	42	undefined
'poisonedByNavigator' in {}	true	false
[].poisonedByScheduler	43	undefined
Object.prototype own poisonedBy*	true	false
getPrototypeOf(navigator) === Object.prototype	true	false

Pinned by PatchingNavigatorsPrototypeDoesNotReachObjectPrototype, the scheduler twin, and GivesEachEngineItsOwnNavigatorPrototype (per-realm).

Cost: +16 B on a default engine

GC.GetAllocatedBytesForCurrentThread over 2,000 constructions, bit-identical on every repeat:

	before	after	Δ
new Engine()	9,256 B	9,272 B	+16 B — two reference fields on Intrinsics
new Engine(o => o.UseWebApis())	24,280 B	24,392 B	+112 B (+0.46%)
UseWebApis(Navigator)	11,176 B	11,288 B	+112 B
UseWebApis() + read navigator.userAgent	27,440 B	27,912 B	+472 B

Nothing materializes until a script names one: CostsNothingUntilItIsRead covers both new names (descriptor still CustomJsValue, _value == null), and NavigatorTests.IsAnEnumerableDataPropertyOfTheGlobal was strengthened to assert the same of the singleton's own descriptor. The +472 B is what a script that actually reads a member pays for a prototype it did not have before.

Verified failing first

Tests written and run against unfixed code: 20 failed / 129 passed, across NavigatorTests ×5, SchedulerTests ×4, InterfaceObjectExposureTests ×9, WebIdlPropertyAttributeTests ×2. Representative:

navigator.userAgent is a WebIDL Attribute and should be
  { writable: n/a, enumerable: true, configurable: true }, but is { … enumerable: false … }
Reflect.ownKeys(navigator).length … to be 0.0 because navigator, but found 2.0
typeof Navigator — actual "undefined", expected "function"

After: 149 / 149.

Suites

Release solution build clean on all five TFMs (1 warning = the pre-existing MSB3277 in Jint.Tests.CommonScripts/net472, identical to baseline).

suite	net10.0	net472
Jint.Tests	10,532 / 4 skipped / 0 failed	7,256 / 4 / 0
Jint.Tests verifying	10,532 / 4 / 0	7,256 / 4 / 0
Jint.Tests.PublicInterface	2,503 / 9 / 0	1,909 / 9 / 0
Jint.Tests.PublicInterface verifying	2,507 / 5 / 0	1,913 / 5 / 0
Jint.Tests.CommonScripts	28 / 0 / 0	28 / 0 / 0
Jint.Tests.SourceGenerators	54 / 0 / 0	—
Jint.Tests.Test262	102,495 passed / 0 failed	—

WPT 422 / 422 both sides, no row moved — and "before" was measured rather than assumed, from a git archive export of the base commit built in a scratch directory. No row could have moved silently: the exclusion table refuses an entry that has started passing and the _minimumTests floors refuse a file that registered fewer, so a green run on both sides is itself the assertion. The corpus census in Vendor/README.md is unchanged. The only vendored files naming Navigator are the two workers/WorkerNavigator* suites, both still excluded under NeedsDeclinedWorkerGlobals.

The WinterTC conformance tables in README.md did not move, checked rather than assumed: §5.1's interface list — transcribed verbatim in WinterTcMinimumCommonApiTests.CommonInterfaces, the pin behind the table — names neither Navigator nor Scheduler, and the only navigator row is §5.2's navigator.userAgent, whose provider is unchanged. Both new names went into the separate per-flag table an embedder actually reads.

Deliberately left undone

• No WorkerNavigator. A worker still gets its parent's navigator; divergences 6 and 7 in Design: Web Workers via a host-supplied worker provider #3167's ledger and their two WPT exclusions stand.
• The [Replaceable] simplification stays — navigator and scheduler remain ordinary enumerable data properties of the global rather than the accessor pair, the same simplification console, crypto, performance and self carry. Node's navigator descriptor is an accessor; that is the one remaining shape difference.
• Scheduler.prototype's brand check is not in APrototypeMemberBrandChecksItsReceiver, because a promise-returning operation rejects rather than throws. The pre-existing SchedulerTests.NeitherOperationEverThrows covers that half, and the omission is documented on the test.

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