AutoCatAvalonia/AutoCat/Windows/MainWindow.axaml.cs

using System;
using System.Linq;
using AutoCat.Pages;
using Avalonia.Controls;
using FluentAvalonia.UI.Controls;

namespace AutoCat.Windows;

public partial class MainWindow : Window
{
    private readonly SettingsPage _settingsPage = new SettingsPage();
    private readonly HistoryPage _historyPage = new HistoryPage();
    
    public MainWindow()
    {
        InitializeComponent();

        NavView.SelectionChanged += NavSelectionChanged;
        NavView.SelectedItem = NavView.MenuItems.First();
    }

    private void NavSelectionChanged(object? sender, NavigationViewSelectionChangedEventArgs args)
    {
        if (args.IsSettingsSelected)
        {
            NavView.Content = _settingsPage;
        }
        else
        {
            NavView.Content = _historyPage;
        }
    }
}