Incorrect controller stack in NavigationStackController when using fluid transitions

[7hommay]

Description

When using fluid transitions it is possible to start presenting a controller while a dismissal of a previous controller is still in progress. Currently the library doesn't handle this correctly. I encountered this issue while working on an app with a similar setup to the StaticNavigationStackController case study.

Checklist

• I have determined whether this bug is also reproducible in a vanilla SwiftUI project.
• If possible, I've reproduced the issue using the main branch of this package.
• This issue hasn't been addressed in an existing GitHub issue or discussion.

Expected behavior

When appending to the path while a dismissal animation is still running, the path should reflect the new target state.

Actual behavior

When appending for example feature2 to the path while a dismissal animation for feature 1 is still running, instead of the path updating to just [feature1] the path updates to [feature1, feature2], thus causing feature1 to be re-presented.

Steps to reproduce

1. Modify the init method of the StaticNavigationStackController case study:

  convenience init(model: Model) {
    @UIBindable var model = model
    self.init(path: $model.path) {
      RootViewController(model: model)
    }
    self.navigationDestination(for: Model.Path.self) { path in
      let controller = {
        switch path {
        case .feature1:
          FeatureViewController(number: 1)
        case .feature2:
          FeatureViewController(number: 2)
        case .feature3:
          FeatureViewController(number: 3)
        }
      }()
      
      if #available(iOS 18, *) {
        controller.preferredTransition = .zoom(options: nil, sourceViewProvider: { _ in
          nil // Not providing a view here for simplicity, the issue can be triggered without
        })
      }
      
      return controller
    }
    self.model = model
  }

2. Push any of the first three features and let the animation settle.
3. Dismiss the opened feature and quickly...
4. ...open a new feature

Simulator.Screen.Recording.-.iPhone.17.Pro.-.2025-11-04.at.14.25.36.mov

SwiftUI Navigation version information

2.6.0

Destination operating system

iOS 18 +

Xcode version information

Version 26.0.1 (17A400)

Swift Compiler version information

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx16.0

[lucamegh]

much needed fix!

[thliu21]

Hi! I took a look at reproducing this and noticed that #243 changes the same pop/path synchronization code. Its current testPopWithReentrantMutation_DoesNotRepush uses a non-animated pop followed by a same-value mutation, while this issue appears to require an iOS 18 fluid/zoom transition with a new destination appended before the dismissal completes.

Is #243 intended to cover this case, or would you be open to a separate deterministic regression test and follow-up fix for that animated in-flight append scenario? I’d be happy to work on it.