Fix font fallback for composite keys containing relative URIs (#14943)

* Add failing test for font fallback with relative URI

* Fix font fallback for composite keys containing relative URIs

src/Avalonia.Base/Media/FontManager.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using Avalonia.Media.Fonts;
@@ -141,22 +142,7 @@ namespace Avalonia.Media
 
         private bool TryGetGlyphTypefaceByKeyAndName(Typeface typeface, FontFamilyKey key, string familyName, [NotNullWhen(true)] out IGlyphTypeface? glyphTypeface)
         {
-            var source = key.Source;
-
-            if (!source.IsAbsoluteUri)
-            {
-                if (key.BaseUri == null)
-                {
-                    throw new NotSupportedException($"{nameof(key.BaseUri)} can't be null.");
-                }
-
-                source = new Uri(key.BaseUri, source);
-            }
-
-            if (source.Scheme == SystemFontScheme)
-            {
-                return SystemFonts.TryGetGlyphTypeface(familyName, typeface.Style, typeface.Weight, typeface.Stretch, out glyphTypeface);
-            }
+            var source = key.Source.EnsureAbsolute(key.BaseUri);
 
             if (TryGetFontCollection(source, out var fontCollection) &&
                 fontCollection.TryGetGlyphTypeface(familyName, typeface.Style, typeface.Weight, typeface.Stretch, out glyphTypeface))
@@ -248,8 +234,9 @@ namespace Avalonia.Media
                 {
                     var key = compositeKey.Keys[i];
                     var familyName = fontFamily.FamilyNames[i];
+                    var source = key.Source.EnsureAbsolute(key.BaseUri);
 
-                    if (TryGetFontCollection(key.Source, out var fontCollection) &&
+                    if (TryGetFontCollection(source, out var fontCollection) &&
                         fontCollection.TryMatchCharacter(codepoint, fontStyle, fontWeight, fontStretch, familyName, culture, out typeface))
                     {
                         return true;
@@ -263,6 +250,8 @@ namespace Avalonia.Media
 
         private bool TryGetFontCollection(Uri source, [NotNullWhen(true)] out IFontCollection? fontCollection)
         {
+            Debug.Assert(source.IsAbsoluteUri);
+
             if (source.Scheme == SystemFontScheme)
             {
                 source = SystemFontsKey;

tests/Avalonia.Skia.UnitTests/Media/FontManagerTests.cs

@@ -200,14 +200,16 @@ namespace Avalonia.Skia.UnitTests.Media
             }
         }
 
-        [Fact]
-        public void Should_Match_Chararcter_Width_Fallbacks()
+        [Theory]
+        [InlineData("NotFound, Unknown", null)] // system fonts
+        [InlineData("/#NotFound, /#Unknown", "avares://some/path")] // embedded fonts
+        public void Should_Match_Character_With_Fallbacks(string familyName, string baseUri)
         {
             using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface.With(fontManagerImpl: new FontManagerImpl())))
             {
                 using (AvaloniaLocator.EnterScope())
                 {
-                    var fontFamily = FontFamily.Parse("NotFound, Unknown");
+                    var fontFamily = FontFamily.Parse(familyName, baseUri is null ? null : new Uri(baseUri));
 
                     Assert.True(FontManager.Current.TryMatchCharacter('A', FontStyle.Normal, FontWeight.Normal, FontStretch.Normal, fontFamily, null, out var typeface));