#appkit: what an app sees on the iPhone Duo simulator

Domain notes for appkit.json (20 829 records, 54 posture runs). Simulator duo-baseline-appkit (E5E4D334-6E6E-4021-A3A4-DA0A743C8B64), iOS 27.1 24A94425, Xcode 27.1. All values are as observed; enums are listed in appkit.json meta.enums.

#Files

Apps: Preferences (SwiftUI SettingsNavigationSplitView), MobileSafari, and VPAppKitProbe, a minimal ObjC UIKit app built for this run (all four orientations in Info.plist, one window scene, UIDeviceFamily 1, a UIHingeInteraction on its root view) that logs every scene, geometry, trait, screen and hinge callback. No Notes / Calendar exists in this runtime (simctl listapps).

#Per app, portrait device orientation

Columns: screen the scene is on, effectiveGeometry.coordinateSpace.bounds, horizontal/vertical size class, interface orientation, window safeAreaInsets (top,left,bottom,right).

• [UIScreen mainScreen] is the cover (LCD, Main) in every posture. [UIScreen screens] has 1 entry when closed and 2 in book/open: the inner UIScreen exists in the app only while the inner panel is lit (UIScreenDidConnectNotification on unfold, …DidDisconnect… on fold). Inner nativeBounds 2007×2853, scale 3; its bounds follow the interface orientation (951×669 landscape, 669×951 portrait). No 669×951-at-boot effect was seen in apps (the README's SpringBoard history effect); the connect notification carries {669, 951} for an instant before the scene settles at 951×669.
• userInterfaceIdiom is Phone (0) everywhere, also on the inner panel with regular/regular.
• The cover status bar is vertical (statusBarAxis 1, UIStatusBarManager.statusBarFrame 466×2), so the cover's safe area is right 84 rather than top. On the inner panel in landscape the probe app and Preferences keep right 84; Safari reports top 82 (its client statusBarAxis 0, horizontal).
• Preferences is portrait-only (supportedInterfaceOrientations mask 2, scene _currentlySupportedInterfaceOrientations 2) yet runs landscapeRight (4) on the inner panel in every posture. Safari and the probe app: scene mask 26 (no upside-down); the probe app's supportedInterfaceOrientationsForWindow is 30.
• Corners: FBSSceneSettings.cornerRadiusConfiguration cover (8, 8, 59, 59), inner 55 on all corners; UITraitCollection.displayCornerRadius and UIScreen._displayCornerRadius are 59 in every posture (_displayCornerRadius does not exist on UITraitCollection).
• ssk_surfaceType (FBSScene settings, ShellSceneKit): cover 1, inner 2, 4 while a launch is still in progress; the probe app does not load ShellSceneKit (does not respond).

#Book vs open

Book and open give the same bounds, size classes and orientation. What differs:

• FBSSceneSettings.sceneRegions: the inner scene has inner {{0,0},{669,951}}, innerRear {{0,0},{669,475.5}}, innerCover {{0,475.5},{669,475.5}}, fold (boundary) {{0,455.5},{669,40}} margins {20,0,20,0}, innerCamera (exclusion, inactive), statusBarOcclusion {{0,0},{124,84}}. In book innerCover is active: YES, in open active: NO. The cover scene has cover, coverCamera, statusBarOcclusion.
• Preferences' split view: primaryColumnWidth 475.67 in book (the sidebar ends at the fold), 320 in open; client primaryWindowOverlayInsets.left 495.67 vs 320. Screenshots: raw/appkit/Preferences/launch-closed/shots/02-book_after-closed_-d3.png and 03-open_after-book_-d3.png.
• Hinge state: _UISceneHingeStateClientComponent.hingeState partiallyOpen 100° / openFlat 180°; scene settings sceneHingeState partiallyOpen / fullyOpen. Only scenes whose client sets wantsSceneHingeState get it: the probe app (has a UIHingeInteraction) and Safari always, Preferences never (nil). The separate FBSSceneSettings.hingeAngle/hingeStatus/ magicPose getters stay 0 in every posture.
• Public UIHingeInteraction: closed status 1 angle 0; book status 2 angle 1.745 (radians); open status 3 angle 3.142. Updates stream during the sweep (10–23 per transition).
• No geometry callback between book and open.

#Device orientation (posture.zsh -deviceOrientation=)

The swap's device orientation reaches apps (UIDevice.orientation and scene orientation):

The inner panel is landscape-native, so a landscape device is a portrait inner panel, and the inner panel allows upside-down even for portrait-only Preferences. On the cover, UIDevice.orientation is the opposite of the requested name (landscapeLeft → 4), on the inner panel it matches (landscapeLeft → 3). Cover landscape statusBarFrame is 0×0.

#Migration and callbacks

• The scene migrates: same UIWindowScene, same persistentIdentifier, same windows; it stays foregroundActive through every fold and unfold (no scene lifecycle callback, no reconnect). Unfold moves it to screens[1] (FBS display Unknown.main[3], hosting space SB-display-<inner UUID>); fold moves it back to Main.
• Probe-app callback order on unfold (book(after-closed)): hinge updates status 1→2, a few deprecated windowScene:didUpdateCoordinateSpace:… calls with no change, then within ~1 ms UIScreenDidConnectNotification → windowScene:didUpdateEffectiveGeometry: (466×678 portrait → 951×669 landscapeRight) → KVO effectiveGeometry → viewWillTransitionToSize → trait change C→R → layout. Geometry lands ~110 ms after the hinge status flips to 2. On fold: trait change R→C first, then didUpdateEffectiveGeometry: (→ 466×678 portrait), viewWillTransitionToSize, then UIScreenDidDisconnectNotification; ~70 ms after the status flips to 1. Exactly one didUpdateEffectiveGeometry: per fold/unfold. The deprecated delegate method still fires (7–12 times per transition). Full sequences: records VPAppKitProbe.callbackSequence, raw logs raw/appkit/VPAppKitProbe/*/events.jsonl.
• UIWindowSceneGeometry.systemFrame is {inf, inf, 0, 0} and UIApplication.statusBarFrame {inf, inf, 0, 0} (recorded as the strings "inf"); UIScreen.applicationFrame has NaN y/height.

#History-dependent values

Same posture, different path (all in appkit.json; compare posture labels):

• FBSSceneClientSettings.interfaceOrientation and UIWindow._sceneClientOrientation: 1 when the app was launched on the cover and unfolded, 4 when it had been on the inner panel first; after folding back to the cover from an inner launch they stay 4 on the cover (portrait UI). Preferences shows the same.
• FBSSceneSettings.statusBarAvoidanceFrame: y 951 after an unfold from closed, 0 otherwise.
• UIWindowScene._hasOrientationDependentSafeArea{Edge,Corner}Insets: true after an unfold from closed, false after an inner launch (book/open).
• Launch timing: 6 s after simctl launch Preferences and Safari launched on the inner panel were still foregroundInactive (activationState 1, deactivationReasons 32, root view 0×0, no scene regions, Safari wantsSceneHingeState NO). The launch-settled scenario repeats both launches with 15 s; use those as the steady state.

#Missing / nil / errors

• nil everywhere: UIWindowScene.sizeRestrictions, .windowingBehaviors, .focusSystem, UITraitCollection.typesettingLanguage, FBSDisplayConfiguration.uniqueIdentifier, FBSSceneSettings.traitCollection, FBSSceneClientSettings.pairedSceneDisplayConfiguration, .requestedDisplayMode, .displayConfigurationRequest.
• nil for some: UISceneConfiguration.name (Preferences, Safari), hinge state (Preferences always; Safari mid-launch), sceneRegions / systemShellHostingSpaceIdentifier mid-launch, UITraitCollection.deviceDeterminedConfiguration (Preferences, Safari).
• does not respond: UITraitCollection._displayCornerRadius (all), and SpringBoard-category getters in apps that do not load those frameworks (ssk_surfaceType, sbsui_surfaceTypeClient, sb_supportsResizing, avkit_screenType, sbh_screenType, sbf_displayProxyIdentifier, pui_fbsDisplayIdentity, isSmallScreenDevice).
• No fold/hinge/posture trait exists on UITraitCollection (method list enumerated).
• UIApplication.currentHingePoseStateForTesting is {0,0,0,0} in every posture.

#Harness notes

• Background apps cannot be probed: the sim suspends them and the lldb expression hangs until the watchdog (a VPAppKitProbe probe timed out this way). Probe only the foreground app.
• simctl launch of an app that already has a process did not bring it to the front here; the driver terminates and relaunches.
• In lldb expressions here @try is unavailable and any ObjC exception aborts the whole expression: FBSSettings throws from valueForKey:, and NSJSONSerialization throws on NaN/inf. The probe calls getters through NSInvocation after respondsToSelector: and turns non-finite numbers into strings. lldb symbol collisions also hit block parameters named d, key and obj; strchr fails to resolve (use NSString).
• A wait loop using pgrep -f <script> matches its own command line; avoid it.