made some minor changes

MainWindow.axaml.cs

@@ -3,74 +3,89 @@ using System.ComponentModel;
 using Avalonia;
 using Avalonia.Controls;
 using System.Reactive.Linq;
+using Avalonia.Input;
+using Avalonia.Interactivity;
 using Avalonia.Media;
 
 namespace SimpleNote;
 
 public partial class MainWindow : Window
 {
-    private MainViewModel ViewModel;
-    public MainWindow(string[]? args)
+    private ViewModel viewModel;
+    private PixelRect? screen;
+    public MainWindow()
     {
-        if (args == null || args.Length < 1)
+        screen = Screens.Primary?.WorkingArea;
+        if (screen == null)
         {
-            Console.WriteLine("Call with Note as Argument!");
+            Console.WriteLine("No screen available.");
             Environment.Exit(1);
         }
+        
+        this.Position = new PixelPoint(screen.Value.Width / 2 - 25, 10);
+        this.Width = 50;
+        // this.MaxWidth = screen.Value.Width - 100;
+        
         InitializeComponent();
-        ViewModel = new MainViewModel();
-        DataContext = ViewModel;
+        viewModel = new ViewModel();
+        DataContext = viewModel;
 
         this.GetObservable(Window.WidthProperty)
             .Subscribe(width =>
             {
-                if (DataContext is MainViewModel vm)
+                if (DataContext is ViewModel vm)
                 {
                     vm.WindowWidth = (double)width;
                 }
             });
-
-        init_TextField(args[0]);
     }
 
+    /*
     public void init_TextField(string text)
     {
         var textBlock = new TextBlock
         {
             Text = text,
-            FontSize = ViewModel.FontSize,
+            FontSize = viewModel.FontSize,
             FontFamily = new FontFamily(FontFamily.DefaultFontFamilyName)
         };
         textBlock.Measure(Size.Infinity);
-        ViewModel.WindowWidth = textBlock.DesiredSize.Width + 50;
+        viewModel.WindowWidth = textBlock.DesiredSize.Width + 50;
         Note.Text = text;
     }
+    */
+
+    public static double TextWidth(string text)
+    {
+        var textBlock = new TextBlock
+        {
+            Text = text,
+            FontSize = ViewModel._fontSize,
+            FontFamily = new FontFamily(FontFamily.DefaultFontFamilyName)
+        };
+        textBlock.Measure(Size.Infinity);
+        return textBlock.DesiredSize.Width + 30 < 60 ? 60 : textBlock.DesiredSize.Width + 30;
+    }
+
+    private void TextBox_OnTextChanged(object? sender, TextChangedEventArgs e)
+    {
+        if (sender == null) return;
+        ((TextBox)sender).Width = TextWidth(((TextBox)sender).Text);
+        double width = 55;
+        foreach (var note in viewModel.Notes)
+        {
+            width += note.Width + 49;
+        }
+
+        if (width == 55) width -= 5;
+        
+        this.Width = width;
+        this.Position = new PixelPoint((int)((screen.Value.Width - width) / 2), 10);
+    }
+
+    private void InputElement_OnKeyDown(object? sender, KeyEventArgs e)
+    {
+        if (e.Key == Key.Enter)
+            this.Focus();
+    }
 }
-
-public class MainViewModel : INotifyPropertyChanged
-{
-    private double _windowWidth = 200;
-    private double _fontSize = 24;
-
-    public double WindowWidth
-    {
-        get => _windowWidth;
-        set
-        {
-            _windowWidth = value;
-            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(WindowWidth)));
-        }
-    }
-
-    public double FontSize
-    {
-        get => _fontSize;
-        set
-        {
-            _fontSize = value;
-            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(FontSize)));
-        }
-    }
-
-    public event PropertyChangedEventHandler? PropertyChanged;
-}