Device: iPhone 11 iOS Version: 17.2.1 Frameworks: UIKit, Auto Layout App Behavior: App supports Arabic (RTL). User can switch language in-app. When language is switched, the app sets UIView.appearance().semanticContentAttribute and fully rebuilds the window’s rootViewController. Problem Summary I update the global semantic direction only when the user explicitly switches language inside the app — e.g.: // Only run when user switches language inside the app UIView.appearance().semanticContentAttribute = .forceRightToLeft // or .forceLeftToRight // then rebuild the window's rootViewController I do not change UIView.appearance().semanticContentAttribute during navigation transitions. Despite that, after switching the language (and rebuilding the root), the app sometimes crashes during a subsequent UINavigationController push/pop animation. The crash appears to be caused by UIKit’s Auto Layout engine removing or updating directional constraints while a navigation transition is running. Crash Log (most relevant portion) Crashed: com.apple.main-thread 0 CoreAutoLayout 0x1372c -[NSISEngine positiveErrorVarForBrokenConstraintWithMarker:errorVar:] + 212 1 CoreAutoLayout 0x121d4 -[NSISEngine removeConstraintWithMarker:] + 1028 2 CoreAutoLayout 0x11d78 -[NSLayoutConstraint _removeFromEngine:] + 148 3 UIKitCore 0x124ba9c __58-[UIView _updateDirectionalConstraintsIfNeededWasFlipped:]_block_invoke_2 + 56 4 UIKitCore 0x484d4 ___UIViewEnumerateLayoutConstraintsAndAdjustForSelectedLayoutVariables_block_invoke + 296 5 UIKitCore 0x4801c -[UIView(AdditionalLayoutSupport) _withUnsatisfiableConstraintsLoggingSuspendedIfEngineDelegateExists:] + 112 6 UIKitCore 0x60830 -[UIView _updateDirectionalConstraintsIfNeededWasFlipped:] + 356 7 UIKitCore 0x60494 -[UIView setSemanticContentAttribute:] + 148 8 CoreFoundation 0x31794 __invoking___ + 148 9 CoreFoundation 0xe6360 -[NSInvocation invokeUsingIMP:] + 332 10 UIKitCore 0x1d93ec __workaround10030904InvokeWithTarget_block_invoke + 68 11 UIKitCore 0x250ec +[UIView _performSystemAppearanceModifications:] + 72 12 UIKitCore 0x3f008 applyInvocationsToTarget + 1004 13 UIKitCore 0x3dcd4 +[_UIAppearance _applyInvocationsTo:window:matchingSelector:onlySystemInvocations:] + 1180 14 UIKitCore 0x3d744 __88-[UIView(Internal) _performUpdatesForPossibleChangesOfIdiom:orScreen:traverseHierarchy:]_block_invoke + 68 15 UIKitCore 0x3d6c4 -[UIView _performUpdatesForPossibleChangesOfIdiom:orScreen:traverseHierarchy:] + 216 16 UIKitCore 0x3d5a0 -[UIView _didChangeFromIdiomOnScreen:traverseHierarchy:] + 112 17 UIKitCore 0x11644 -[UIView(Internal) _didMoveFromWindow:toWindow:] + 1220 18 UIKitCore 0x1142c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 684 19 UIKitCore 0x1142c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 684 20 UIKitCore 0x1142c -[UIView(Internal) _didMoveFromWindow:toWindow:] + 684 21 UIKitCore 0x10eb4 __45-[UIView(Hierarchy) _postMovedFromSuperview:]_block_invoke + 124 22 CoreAutoLayout 0xa514 -[NSISEngine withBehaviors:performModifications:] + 84 23 UIKitCore 0x10ddc -[UIView _postMovedFromSuperview:] + 504 24 UIKitCore 0xfa24 -[UIView(Internal) _addSubview:positioned:relativeTo:] + 2200 25 UIKitCore 0x7a63b8 __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke_2 + 1252 26 UIKitCore 0x41a70 +[UIView(Animation) performWithoutAnimation:] + 76 27 UIKitCore 0x7a5e84 __53-[_UINavigationParallaxTransition animateTransition:]_block_invoke + 240 28 UIKitCore 0x12749c +[UIView _performBlockDelayingTriggeringResponderEvents:forScene:] + 176 29 UIKitCore 0x7a5990 -[_UINavigationParallaxTransition animateTransition:] + 952 30 UIKitCore 0x291098 ___UIViewControllerTransitioningRunCustomTransition_block_invoke_3 + 52 31 UIKitCore 0x29100c +[UIKeyboardSceneDelegate _pinInputViewsForKeyboardSceneDelegate:onBehalfOfResponder:duringBlock:] + 96 32 UIKitCore 0x290f70 ___UIViewControllerTransitioningRunCustomTransition_block_invoke_2 + 196 33 UIKitCore 0x1d8c8c +[UIView(Animation) _setAlongsideAnimations:toRunByEndOfBlock:] + 180 34 UIKitCore 0x1d851c _UIViewControllerTransitioningRunCustomTransition + 484 35 UIKitCore 0x6f5a84 -[UINavigationController _startCustomTransition:] + 3292 36 UIKitCore 0x1182a8 -[UINavigationController _startDeferredTransitionIfNeeded:] + 496 37 UIKitCore 0x1179a0 -[UINavigationController __viewWillLayoutSubviews] + 96 38 UIKitCore 0x117904 -[UILayoutContainerView layoutSubviews] + 172 39 UIKitCore 0x3297c -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1528 40 QuartzCore 0x66aa8 CA::Layer::layout_if_needed(CA::Transaction*) + 500 41 QuartzCore 0x66630 CA::Layer::layout_and_display_if_needed(CA::Transaction*) + 144 42 QuartzCore 0x6cb60 CA::Context::commit_transaction(CA::Transaction*, double, double*) + 464 43 QuartzCore 0x65e3c CA::Transaction::commit() + 648 44 QuartzCore 0x65ae4 CA::Transaction::flush_as_runloop_observer(bool) + 88 45 CoreFoundation 0x3583c __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 36 46 CoreFoundation 0x34244 __CFRunLoopDoObservers + 548 47 CoreFoundation 0x33960 __CFRunLoopRun + 1028 48 CoreFoundation 0x33478 CFRunLoopRunSpecific + 608 49 GraphicsServices 0x34f8 GSEventRunModal + 164 50 UIKitCore 0x22c62c -[UIApplication _run] + 888 51 UIKitCore 0x22bc68 UIApplicationMain + 340 52 MyApp 0x46a040 main + 44 (AppDelegate.swift:44) 53 ??? 0x1c8d52dcc (缺少) Questions / Requests Recommended patterns for in-app language switching (LTR ⇄ RTL) to avoid direction/constraint races during animations? For example: Should semantic direction be applied to the window/root view controller only? Should we avoid rebuilding root during an active transition? Any suggested synchronization (e.g., wait for transitions to finish) or APIs to call after rebuilding?