NavigationLink double click on List for MacOS

I had the same question and looked around but couldn't find a gesture based solution which works in all cases, especially not with List that has selection states. I made a view modifier using an NSEvent click monitor which seems to solve it as expected for all cases I've tried:

extension View {
    /// Adds a double click handler this view (macOS only)
    ///
    /// Example
    /// ```
    /// Text("Hello")
    ///     .onDoubleClick { print("Double click detected") }
    /// ```
    /// - Parameters:
    ///   - inset: To shrink or expand the double click hit area of the view, pass positive or negative values, respectively
    ///   - handler: Block invoked when a double click is detected
    func onDoubleClick(inset: CGSize = .zero, handler: @escaping () -> Void) -> some View {
        modifier(DoubleClickHandler(inset: inset, handler: handler))
    }
}

struct DoubleClickHandler: ViewModifier {
    let inset: CGSize
    let handler: () -> Void
    @State private var monitor: Any?
​
    private func addListener(frame: NSRect) {
        let adjustedFrame = frame.insetBy(dx: inset.width, dy: inset.height)
        monitor = NSEvent.addLocalMonitorForEvents(matching: .leftMouseDown) {
            if $0.clickCount == 2 {
                var location = $0.locationInWindow
                location.y = $0.window!.frame.size.height - location.y // Flip to fix coordinate system mismatch
                if adjustedFrame.contains(location) { handler() }
            }
            return $0
        }
    }

    private func removeListener()  {
        if let monitor = monitor {
            NSEvent.removeMonitor(monitor)
        }
    }

    func body(content: Content) -> some View {
        content.background {
            GeometryReader { proxy in
                Color.clear
                    .onAppear { addListener(frame: proxy.frame(in: .global)) }
                    .onDisappear { removeListener() }
            }
        }
    }
}

https://gist.github.com/joelekstrom/91dad79ebdba409556dce663d28e8297

Actually, the code pasted in the answer above is now outdated - check the gist instead. The original code had some issues when list was scrolled. Updated gist uses an NSView to detect the clicks instead

extension View {
    /// Adds a double click handler this view (macOS only)
    ///
    /// Example
    /// ```
    /// Text("Hello")
    ///     .onDoubleClick { print("Double click detected") }
    /// ```
    /// - Parameters:
    ///   - handler: Block invoked when a double click is detected
    func onDoubleClick(handler: @escaping () -> Void) -> some View {
        modifier(DoubleClickHandler(handler: handler))
    }
}

struct DoubleClickHandler: ViewModifier {
    let handler: () -> Void
    func body(content: Content) -> some View {
        content.overlay {
            DoubleClickListeningViewRepresentable(handler: handler)
        }
    }
}

struct DoubleClickListeningViewRepresentable: NSViewRepresentable {
    let handler: () -> Void
    func makeNSView(context: Context) -> DoubleClickListeningView {
        DoubleClickListeningView(handler: handler)
    }
    func updateNSView(_ nsView: DoubleClickListeningView, context: Context) {}
}

class DoubleClickListeningView: NSView {
    let handler: () -> Void

    init(handler: @escaping () -> Void) {
        self.handler = handler
        super.init(frame: .zero)
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func mouseDown(with event: NSEvent) {
        super.mouseDown(with: event)
        if event.clickCount == 2 {
            handler()
        }
    }
}

I had the same problem in a NavigationSplitView and managed to solve it using simultaneous gestures as you originally tried. To make sure single-clicking fully matches native behaviour, my single-click handler sets the selection which is passed as a binding to the List, and it also sets a FocusState.

struct ContentView: View {
    // Assume there's an "items" array of Item instances to display.

    @State private var selectedItem: Item?
    @FocusState var listFocused: Bool
    
    var body: some View {
        NavigationSplitView {
            List(selection: $selectedItem) {
                ForEach(items) { item in
                    NavigationLink(value: item) { Text(verbatim: item.name) }
                        .gesture(doubleClickGesture(for: item))
                }
            }
            .focused($listFocused)
        } detail: {
            DetailView(item)
        }
    }
    
    private func doubleClickGesture(for item: Item) -> some Gesture {
        SimultaneousGesture(
            TapGesture(count: 1).onEnded {
                selectedItem = item
                listFocused = true
            },
            TapGesture(count: 2).onEnded {
                print(“Double-clicked”)
            }
        )
    }
}