FINALLY found the multi-char disappearing char bug!

I need to actually treat Symbol Empty and Symbol space different, as Symbol space is for drawing background, and Symbol.Empty represents pixel that is invisible because
of prior double-wide char overlapping with it.

src/Consolonia.Core/Drawing/DrawingContextImpl.cs

@@ -175,7 +175,7 @@ public void DrawGeometry(IBrush brush, IPen pen, IGeometryImpl geometry)
                         CurrentClip.ExecuteWithClipping(new Point(px, py), () =>
                         {
                             _pixelBuffer.Set(new PixelBufferCoordinate((ushort)px, (ushort)py),
-                                (pixel, bb) => (pixel ?? new Pixel()).Blend(new Pixel(new PixelBackground(bb.Mode, bb.Color))),
+                                (pixel, bb) => pixel.Blend(new Pixel(new PixelBackground(bb.Mode, bb.Color))),
                                 backgroundBrush);
                         });
                     }
@@ -539,8 +539,21 @@ private void DrawPixelAndMoveHead(ref Point head, Line line, LineStyle? lineStyl
                                 _pixelBuffer.Set((PixelBufferCoordinate)characterPoint,
                                     (oldPixel, cp) => oldPixel.Blend(cp), consolePixel);
                             });
-
                             currentXPosition++;
+                            if (symbol.Width > 1)
+                            {
+                                for(int i=1;i<symbol.Width;i++)
+                                {
+                                    characterPoint =
+                                        whereToDraw.Transform(Matrix.CreateTranslation(currentXPosition, currentYPosition));
+                                    CurrentClip.ExecuteWithClipping(characterPoint, () =>
+                                    {
+                                        _pixelBuffer.Set((PixelBufferCoordinate)characterPoint,
+                                            (oldPixel, cp) => Pixel.Empty, consolePixel);
+                                    });
+                                    currentXPosition++;
+                                }
+                            }
                         }
                         break;
                 }

src/Consolonia.Core/Drawing/PixelBufferImplementation/Pixel.cs

@@ -11,21 +11,26 @@ namespace Consolonia.Core.Drawing.PixelBufferImplementation
     [DebuggerDisplay("'{Foreground.Symbol.Text}' [{Foreground.Color}, {Background.Color}]")]
     public class Pixel : IEquatable<Pixel>
     {
+        // Pixel empty is a non-pixel. It has no symbol, no color, no weight, no style, no text decoration, and no background.
+        // it is used only when a multichar sequence overlaps a pixel making it a non-entity.
         public static Pixel Empty => new Pixel();
 
+        // pixel space is a pixel with a space symbol, but could have color blended into. it is used to advance the cursor
+        // and set the background color
+        public static Pixel Space => new Pixel(new PixelForeground(new SimpleSymbol(' '), Colors.Transparent), new PixelBackground(Colors.Transparent));
+
         public PixelForeground Foreground { get; set; }
 
         public PixelBackground Background { get; set; }
 
         public bool IsCaret { get; set; }
 
-        public Pixel()
+        protected Pixel()
         {
             Foreground = new PixelForeground();
             Background = new PixelBackground();
             IsCaret = false;
         }
-
         public Pixel(bool isCaret)
         {
             Foreground = new PixelForeground();
@@ -56,7 +61,7 @@ public Pixel(ISymbol symbol,
         /// </summary>
         /// <param name="background"></param>
         public Pixel(PixelBackground background) :
-            this(new PixelForeground(new SimpleSymbol(), Colors.Transparent),
+            this(new PixelForeground(new SimpleSymbol(' '), Colors.Transparent),
                 background)
         {
         }

src/Consolonia.Core/Drawing/PixelBufferImplementation/PixelBuffer.cs

@@ -21,9 +21,12 @@ public PixelBuffer(ushort width, ushort height)
             Width = width;
             Height = height;
             _buffer = new Pixel[width, height];
+
+            // initialize the buffer with space so it draws any background color
+            // blended into it.
             for (ushort y = 0; y < height; y++)
                 for (ushort x = 0; x < width; x++)
-                    _buffer[x, y] = new Pixel();
+                    _buffer[x, y] = Pixel.Space;
         }
 
         public ushort Width { get; }

src/Consolonia.Core/Drawing/PixelBufferImplementation/SimpleSymbol.cs

@@ -13,9 +13,9 @@ public class SimpleSymbol : ISymbol, IEquatable<SimpleSymbol>
     {
         public SimpleSymbol()
         {
-            // we use String.Empty to represent an empty symbol. It still takes up space, but it's invisible
+            // we use String.Empty to represent an empty symbol
             Text = string.Empty;
-            Width = 1;
+            Width = 0;
         }
 
         public SimpleSymbol(char character)

src/Consolonia.Core/Drawing/RenderTarget.cs

@@ -87,9 +87,11 @@ private void OnResized(Size size, WindowResizeReason reason)
         private static Pixel?[,] InitializeCache(ushort width, ushort height)
         {
             var cache = new Pixel?[width, height];
+
+            // initalize the cache with Pixel.Empty as it literally means nothing
             for (ushort y = 0; y < height; y++)
                 for (ushort x = 0; x < width; x++)
-                    cache[x, y] = new Pixel();
+                    cache[x, y] = Pixel.Empty;
             return cache;
         }
 
@@ -192,9 +194,10 @@ public void WritePixel(
                     _lastBufferPointStart = _currentBufferPoint = bufferPoint;
                 }
 
-                if ((pixel.Foreground.Symbol?.Text.Length ?? 0) == 0)
-                    _stringBuilder.Append(' ');
-                else
+                // the only pixels without width are Empty pixels, which we don't 
+                // want to output as they are already invisible and represented
+                // by the complex glyph coming before it (aka double-wide chars)
+                if (pixel.Foreground.Symbol.Width > 0)
                     _stringBuilder.Append(pixel.Foreground.Symbol!.Text);
 
                 _currentBufferPoint = _currentBufferPoint.WithXpp();

src/Consolonia.Core/Infrastructure/ConsoleWindow.cs

@@ -299,7 +299,7 @@ private void OnConsoleOnResized()
         {
             // clear screen so we don't see crazy while resizing.
 #pragma warning disable CA1303 // Do not pass literals as localized parameters
-            System.Console.Write(ConsoleUtils.ClearScreen);
+            System.Console.Write(ESC.ClearScreen);
 #pragma warning restore CA1303 // Do not pass literals as localized parameters
 
             // Cancel previous task if there is one and start a new one

src/Consolonia.Core/Infrastructure/InputLessDefaultNetConsole.cs

@@ -30,9 +30,9 @@ private void PrepareConsole()
         {
 #pragma warning disable CA1303 // Do not pass literals as localized parameters
             // enable alternate screen so original console screen is not affected by the app
-            WriteText(ConsoleUtils.EnableAlternateBuffer);
-            WriteText(ConsoleUtils.HideCursor);
-            WriteText(ConsoleUtils.ClearScreen);
+            WriteText(ESC.EnableAlternateBuffer);
+            WriteText(ESC.HideCursor);
+            WriteText(ESC.ClearScreen);
 #pragma warning restore CA1303 // Do not pass literals as localized parameters
         }
 
@@ -47,7 +47,7 @@ public bool CaretVisible
             set
             {
                 if (_caretVisible == value) return;
-                WriteText(value ? ConsoleUtils.ShowCursor : ConsoleUtils.HideCursor);
+                WriteText(value ? ESC.ShowCursor : ESC.HideCursor);
                 _caretVisible = value;
             }
 #pragma warning restore CA1303 // Do not pass literals as localized parameters
@@ -74,7 +74,7 @@ public bool SupportsComplexEmoji
                         // TODO, escape sequence
                         var (left2, _) = Console.GetCursorPosition();
                         _supportEmoji = left2 - left == 2;
-                        WriteText(ConsoleUtils.SetCursorPosition(left, top));
+                        WriteText(ESC.SetCursorPosition(left, top));
                     }
                     catch (Exception)
                     {
@@ -112,7 +112,7 @@ public void SetCaretPosition(PixelBufferCoordinate bufferPoint)
 
             try
             {
-                WriteText(ConsoleUtils.SetCursorPosition(bufferPoint.X, bufferPoint.Y));
+                WriteText(ESC.SetCursorPosition(bufferPoint.X, bufferPoint.Y));
             }
             catch (ArgumentOutOfRangeException argumentOutOfRangeException)
             {
@@ -134,17 +134,17 @@ public void Print(PixelBufferCoordinate bufferPoint, Color background, Color for
 
             var sb = new StringBuilder();
             if (textDecoration == TextDecorationLocation.Underline)
-                sb.Append(ConsoleUtils.Underline);
+                sb.Append(ESC.Underline);
 
             if (textDecoration == TextDecorationLocation.Strikethrough)
-                sb.Append(ConsoleUtils.Strikethrough);
+                sb.Append(ESC.Strikethrough);
 
             if (style == FontStyle.Italic)
-                sb.Append(ConsoleUtils.Italic);
+                sb.Append(ESC.Italic);
 
-            sb.Append(ConsoleUtils.Background(background));
+            sb.Append(ESC.Background(background));
 
-            sb.Append(ConsoleUtils.Foreground(weight switch
+            sb.Append(ESC.Foreground(weight switch
             {
                 FontWeight.Medium or FontWeight.SemiBold or FontWeight.Bold or FontWeight.ExtraBold or FontWeight.Black
                     or FontWeight.ExtraBlack
@@ -155,7 +155,7 @@ FontWeight.Thin or FontWeight.ExtraLight or FontWeight.Light
             }));
 
             sb.Append(str);
-            sb.Append(ConsoleUtils.Reset);
+            sb.Append(ESC.Reset);
 
             WriteText(sb.ToString());
 
@@ -190,8 +190,8 @@ public void Dispose()
         {
             Dispose(true);
             GC.SuppressFinalize(this);
-            WriteText(ConsoleUtils.DisableAlternateBuffer);
-            WriteText(ConsoleUtils.ShowCursor);
+            WriteText(ESC.DisableAlternateBuffer);
+            WriteText(ESC.ShowCursor);
         }
 #pragma warning restore CA1063 // Implement IDisposable Correctly
 #pragma warning restore CA1303 // Do not pass literals as localized parameters

src/Consolonia.Core/Text/ConsoleUtils.cs → src/Consolonia.Core/Text/ESC.cs

@@ -5,7 +5,7 @@ namespace Consolonia.Core.Text
     /// <summary>
     ///     ANSI escape definitions and utility methods.
     /// </summary>
-    internal static class ConsoleUtils
+    internal static class ESC
     {
         // Control
         public const string EndOfText = "\u0003";
@@ -31,6 +31,14 @@ internal static class ConsoleUtils
         public const string HideCursor = "\u001b[?25l";
         public const string ShowCursor = "\u001b[?25h";
 
+        // move cursor
+        public static string MoveCursorUp(int n) => $"\u001b[{n}A";
+
+        public static string MoveCursorDown(int n) => $"\u001b[{n}B";
+
+        public static string MoveCursorRight(int n) => $"\u001b[{n}C";
+
+        public static string MoveCursorLeft(int n) => $"\u001b[{n}D";
 
         public static string SetCursorPosition(int x, int y) 
             => $"\u001b[{y+1};{x+1}f";

src/Consolonia.Gallery/Gallery/GalleryViews/GalleryWelcome.axaml

@@ -2,9 +2,31 @@
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
              xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
-             mc:Ignorable="d" 
+             mc:Ignorable="d"
              x:Class="Consolonia.Gallery.Gallery.GalleryViews.GalleryWelcome">
-    <TextBlock FontWeight="Bold" TextDecorations="Underline" HorizontalAlignment="Center" >
-        Welcome to Consolonia!
-    </TextBlock>
+    <StackPanel>
+        <!--This binds to the observable collection in the ViewModel-->
+        <ItemsControl ItemsSource="{Binding Glyphs}">
+
+            <!--Use a StackPanel to display all the modules-->
+            <ItemsControl.ItemsPanel>
+                <ItemsPanelTemplate>
+                    <StackPanel Orientation="Horizontal"/>
+                </ItemsPanelTemplate>
+            </ItemsControl.ItemsPanel>
+
+            <!--This defines the layout for each item (i.e. each module)-->
+            <ItemsControl.ItemTemplate>
+                <DataTemplate>
+                    <!--The controls to display each item; bind them to properties on the HardwareModule class-->
+                    <TextBlock Text="{Binding}" />
+                </DataTemplate>
+            </ItemsControl.ItemTemplate>
+        </ItemsControl>
+        <StackPanel Orientation="Horizontal" HorizontalAlignment="Left">
+            <Button Content="Add" Click="OnAdd"/>
+            <Button Content="Remove" Click="OnRemove"/>
+        </StackPanel>
+    </StackPanel>
+
 </UserControl>

src/Consolonia.Gallery/Gallery/GalleryViews/GalleryWelcome.axaml.cs

@@ -1,5 +1,7 @@
+using System.Collections.ObjectModel;
 using Avalonia.Controls;
 using Avalonia.Markup.Xaml;
+using CommunityToolkit.Mvvm.ComponentModel;
 
 namespace Consolonia.Gallery.Gallery.GalleryViews
 {
@@ -8,12 +10,34 @@ public partial class GalleryWelcome : UserControl
     {
         public GalleryWelcome()
         {
+            this.DataContext = new TestModel();
             InitializeComponent();
         }
 
         private void InitializeComponent()
         {
             AvaloniaXamlLoader.Load(this);
         }
+
+        private void OnAdd(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            var model = (TestModel)this.DataContext;
+            model.Glyphs.Insert(0, "X");
+        }
+
+        private void OnRemove(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
+        {
+            var model = (TestModel)this.DataContext;
+            model.Glyphs.RemoveAt(0);
+        }
+    }
+
+    public partial class TestModel : ObservableObject
+    {
+        [ObservableProperty]
+        private ObservableCollection<string> glyphs = new ObservableCollection<string>()
+        {
+            "中"
+        };
     }
 }

src/Tests/Consolonia.Core.Tests/PixelBufferTests.cs

@@ -18,7 +18,7 @@ private static PixelBuffer CreateBuffer()
                     if (x % 3 == 0)
                         buffer[x, y] = new Pixel(new SimpleSymbol($"{x},{y}"), Colors.Blue);
                     else if (x % 3 == 1)
-                        buffer[x, y] = new Pixel();
+                        buffer[x, y] = Pixel.Empty;
                     else
                         buffer[x, y] = new Pixel(new SimpleSymbol($"{x},{y}"), Colors.White, FontStyle.Italic, FontWeight.Bold, TextDecorationLocation.Underline);
                 }

src/Tools/Consolonia.PreviewHost/Properties/launchSettings.json

@@ -6,7 +6,7 @@
     },
     "Consolonia.PreviewHost": {
       "commandName": "Project",
-      "commandLineArgs": "GalleryTextBlock.axaml",
+      "commandLineArgs": "GalleryWelcome.axaml",
       "workingDirectory": "S:\\github\\Consolonia\\src\\Consolonia.Gallery"
     }
   }