add headless fontmanager.

src/Avalonia.Headless/AvaloniaHeadlessPlatform.cs

@@ -65,7 +65,8 @@ namespace Avalonia.Headless
                 .Bind<IKeyboardDevice>().ToConstant(new KeyboardDevice())
                 .Bind<IRenderLoop>().ToConstant(new RenderLoop())
                 .Bind<IRenderTimer>().ToConstant(new RenderTimer(60))
-
+                .Bind<IFontManagerImpl>().ToSingleton<HeadlessFontManagerStub>()
+                .Bind<ITextShaperImpl>().ToSingleton<HeadlessTextShaperStub>()
                 .Bind<IWindowingPlatform>().ToConstant(new HeadlessWindowingPlatform())
                 .Bind<PlatformHotkeyConfiguration>().ToSingleton<PlatformHotkeyConfiguration>();
         }

src/Avalonia.Headless/HeadlessPlatformRenderInterface.cs

@@ -104,6 +104,11 @@ namespace Avalonia.Headless
 
             public Rect GetRenderBounds(IPen pen)
             {
+                if(pen is null)
+                {
+                    return Bounds;
+                }
+
                 return Bounds.Inflate(pen.Thickness / 2);
             }
 

src/Avalonia.Headless/HeadlessPlatformStubs.cs

@@ -1,12 +1,18 @@
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls;
 using Avalonia.Controls.Platform;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
+using Avalonia.Media;
+using Avalonia.Media.Fonts;
+using Avalonia.Media.TextFormatting;
 using Avalonia.Platform;
+using Avalonia.Utility;
 
 namespace Avalonia.Headless
 {
@@ -74,6 +80,88 @@ namespace Avalonia.Headless
         }
     }
 
+    class HeadlessGlyphTypefaceImpl : IGlyphTypefaceImpl
+    {
+        public short DesignEmHeight => 10;
+
+        public int Ascent => 5;
+
+        public int Descent => 5;
+
+        public int LineGap => 2;
+
+        public int UnderlinePosition => 5;
+
+        public int UnderlineThickness => 5;
+
+        public int StrikethroughPosition => 5;
+
+        public int StrikethroughThickness => 2;
+
+        public bool IsFixedPitch => true;
+
+        public void Dispose()
+        {            
+        }
+
+        public ushort GetGlyph(uint codepoint)
+        {
+            return 1;
+        }
+
+        public int GetGlyphAdvance(ushort glyph)
+        {
+            return 1;
+        }
+
+        public int[] GetGlyphAdvances(ReadOnlySpan<ushort> glyphs)
+        {
+            return glyphs.ToArray().Select(x => (int)x).ToArray();
+        }
+
+        public ushort[] GetGlyphs(ReadOnlySpan<uint> codepoints)
+        {
+            return codepoints.ToArray().Select(x => (ushort)x).ToArray();
+        }
+    }
+
+    class HeadlessTextShaperStub : ITextShaperImpl
+    {
+        public GlyphRun ShapeText(ReadOnlySlice<char> text, TextFormat textFormat)
+        {
+            return new GlyphRun(new GlyphTypeface(new Typeface("Arial")), textFormat.FontRenderingEmSize,
+                new ReadOnlySlice<ushort>(new ushort[] { 1, 2, 3 }),
+                new ReadOnlySlice<double>(new double[] { 1, 2, 3 }),
+                new ReadOnlySlice<Vector>(new Vector[] { new Vector(1, 1), new Vector(2, 2), new Vector(3, 3) }),
+                text,
+                new ReadOnlySlice<ushort>(new ushort[] { 1, 2, 3 }));
+        }
+    }
+
+    class HeadlessFontManagerStub : IFontManagerImpl
+    {
+        public IGlyphTypefaceImpl CreateGlyphTypeface(Typeface typeface)
+        {
+            return new HeadlessGlyphTypefaceImpl();
+        }
+
+        public string GetDefaultFontFamilyName()
+        {
+            return "Arial";
+        }
+
+        public IEnumerable<string> GetInstalledFontFamilyNames(bool checkForUpdates = false)
+        {
+            return new List<string> { "Arial" };
+        }
+
+        public bool TryMatchCharacter(int codepoint, FontWeight fontWeight, FontStyle fontStyle, FontFamily fontFamily, CultureInfo culture, out FontKey fontKey)
+        {
+            fontKey = new FontKey("Arial", fontWeight, fontStyle);
+            return true;
+        }
+    }
+
     class HeadlessIconLoaderStub : IPlatformIconLoader
     {