MercuryConverter/UI/MainWindow.axaml.cs

using System;
using System.Threading.Tasks;
using Avalonia.Controls;
using Avalonia.Styling;
using Avalonia.Threading;
using MercuryConverter.UI.Dialogs;
using MercuryConverter.UI.Views;

namespace MercuryConverter.UI;

public partial class MainWindow : Window
{
    public static MainWindow? Instance { get; private set; }
    private bool initialShown = false;

    public MainWindow()
    {
        Instance = this;
        InitializeComponent();

        // Force dark mode in designer
        if (Design.IsDesignMode)
        {
            RequestedThemeVariant = ThemeVariant.Dark;
        }

        // Setup tab views
        SelectionControl.Content = new Selection();

        // Show dialog on startup
        Activated += OnActivated;
    }

    private void OnActivated(object? sender, EventArgs e)
    {
        if (initialShown) return;
        initialShown = true;

        Dialog.DialogContent = new Welcome().Content;
        Dialog.IsOpen = true;
    }
}