canShip requires build ≥ 400.

A unit harness asserts both 418 and 200. A shallow UI harness only asks whether a Ship button exists. This bench runs the same gate under three scenarios and shows who blinks.

Filmstrip: HEALTHY both green, VM BUG UNIT red UI green, UI BUG label fail.

Healthy — both agree

ShipmentGate.canShip(build: 418) // true
ShipmentGate.canShip(build: 200) // false

SCENARIO HEALTHY. UNIT 2/2 all pass. UI 2/2 all pass. canShip(418) true, canShip(200) false. Ship 418 enabled, Ship 200 disabled.

Harness	Score	Notes
UNIT	2/2	418 true, 200 false
UI	2/2	button exists + a11y label

Sample UI: Ship 418 enabled, Ship 200 disabled.

VM bug — unit fails, UI stays green

Invert the gate. Build 418 wrongly refuses; build 200 wrongly ships.

SCENARIO VM BUG. UNIT 0/2. UI 2/2. canShip(418) got false want true; canShip(200) got true want false. Ship 418 disabled, Ship 200 enabled. Note: UI stayed green while UNIT failed.

Harness	Score	Notes
UNIT	0/2	both assertions fail
UI	2/2	still finds Ship + label

That green UI column is the control: existence checks do not see inverted logic. Sample UI shows the lie — Ship 200 lit, Ship 418 dark.

XCUITest agreed with the shallow harness

Reusing the ch8 LaunchLabAXTests target:

testShipButtonExists_vmBug_stillFindsButton passed

It launched -mode vm, waited for ship-418, and passed — same miss the on-screen UI column prints.

UI bug — unit green, label fails

Logic restored; accessibility label stripped.

SCENARIO UI BUG. UNIT 2/2. UI 1/2. a11y label FAIL unlabelled. UNIT stayed green; UI caught the missing label.

Harness	Score	Notes
UNIT	2/2	gate sound again
UI	1/2	button exists PASS, a11y label FAIL

Unit tests do not open VoiceOver's tree. A labelled control is a UI concern — ch8's lesson, reused here.

Pyramid, photographed

TDD books split unit / integration / UI. This still is the split as scoreboards: UNIT catches the inverted ViewModel; UI catches the missing label; neither catches everything alone.

Mini-exercise

Add testCanShip200_vmMode that launches -mode vm and asserts Ship 200 is not enabled. Watch it fail today — that is the missing assertion the shallow harness skipped.

Challenges

Add a LaunchLabUnitTests target that imports ShipmentGate and asserts the healthy matrix without the bench UI.
Extend the UI test to assert Ship 200 is not hittable in healthy mode.
Inject a network client protocol and show a mock unit test vs a live UI smoke.
Make the UI harness read the on-screen PASS/FAIL labels via XCUIApplication().staticTexts.
Wire CI to run -only-testing:LaunchLabAXTests/ShipmentGateUITests on the LaunchLab simulator.

Key Points

Observed HEALTHY: UNIT 2/2, UI 2/2.
Observed VM BUG: UNIT 0/2, UI 2/2 — control miss.
Observed UI BUG: UNIT 2/2, UI 1/2 (label).
ShipmentGate.canShip is pure — unit tests do not need a simulator.
Shallow UI tests can pass while the ViewModel lies.
Real ShipmentGateUITests still found ship-418 under -mode vm.
Accessibility labels are UI-test territory (ch8 harness reused).
Use both layers; photograph what each refuses to see.

Next up: dependency injection — one screen wired to a live service, a stub, and a failing stub, switchable by launch argument.

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What the Tests Catch