I’ve caught a very strange problem (THAT ONLY OCCURS ON REAL DEVICES) that has to do with using @NameSpace and MatchedGeometryEffect using NavigationStack. The problem is that if I go from the first screen to the second screen (on the second screen I have a View with MatchedGeometryEffect) , I get a strange bug where the MatchedGeometryEffect animation starts to work very badly (ONLY ON REAL DEVICE), it feels like the animation frame count drops to a minimum, but on simulator or preview everything works fine as it should. However, if I use the screen without NavigationStack, there is no such animation problem. What can this be related to ? And how can this be fixed ? It only takes a couple of lines of code to catch the animation problem, but it took all day to figure out what the problem is.

FirstView

struct NameSpaceTest2Navigation: View {

    @State private var nameSpaceTest2Path: [String] = []

    var body: some View {
        NavigationStack(path: $nameSpaceTest2Path) {
            Button(action: {
                nameSpaceTest2Path.append("nameSpaceTest2")
            }, label: {
                Text("Button")
            })
            .navigationDestination(for: String.self) { path in
                NameSpaceTest2()
            }
        }
    }
}

SecondView

struct NameSpaceTest2: View {
    @State private var selection: Int = 0

    var body: some View {
        VStack {

            TabView(selection: $selection) {
                ForEach(0..<5, id: .self) { _ in
                    Color.white
                }
            }
            .tabViewStyle(.page(indexDisplayMode: .never))
        }
        .overlay(alignment: .top) {
            NameSpaceTest2Header(selection: $selection)
        }
    }
}

ThirdView

struct NameSpaceTest2Header: View {
    @Binding var selection: Int

    @Namespace var sectionUnderline

    var body: some View {
        ScrollViewReader { scrollReader in //START: ScrollViewReader
            ScrollView(.horizontal, showsIndicators: false) { //START: ScrollView
                HStack(spacing: 0) { //START: HStack
                    ForEach(0..<5, id: .self) { index in
                        VStack { //START: VStack
                            Text("Name: (index)")
                                .foregroundStyle(Color.green)
                                .padding(.horizontal, 24)
                                .padding(.vertical, 15)
                                .overlay(alignment: .bottom) {
                                    if selection == index {
                                        Rectangle()
                                            .fill(Color.red)
                                            .frame(height: 5)
                                            .matchedGeometryEffect(id: "sectionUnderline", in: sectionUnderline, properties: .frame)
                                            .transition(.scale(scale: 1))
                                    }
                                }
                                .animation(.smooth, value: selection)
                        } //END: VStack
                        .onTapGesture {
                            withAnimation(.smooth) {
                                scrollReader.scrollTo(index)
                                selection = index
                            }
                        }
                        .tag(index)
                    }
                } //END: HStack
            } //END: ScrollView
            .onChange(of: selection, perform: { value in
                withAnimation(.smooth) {
                    scrollReader.scrollTo(value, anchor: .center)
                }
            })
        } //END: ScrollViewReader
    }
}

Try to repeat this code and run it on a real device with NavigationStack and you will get a bug with the animation, it will twitch like it has 5-10 frames per second.

Then try to run Second View without Navigation Stack and you will get smooth animation, which is what it should be.

What could be the problem ? How to get the smooth animation back ?

Once again, you should only test on a real device.

I tried wrapping selection = index in DispacthQueue.main.ascync , but it didn’t work. I tried changing the animation, but that didn’t work either. If I set the animation, I can see how the Rectangle changes size almost frame by frame.
Checked the performance in Instruments, nothing highly loaded is happening.
Didn’t expect to have a problem with this.
Can’t imagine what the problem could be, I expected the animation to be smooth, just like on the simulator