Draw Han in the UI language's own font

src/Filee.App/Program.cs

@@ -7,9 +7,12 @@
 //  3. Start Avalonia. The app lives in the tray, so it only exits via the tray "Quit" command (or --quit).
 
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Media;
+using Avalonia.Media.Fonts;
 using Filee.App.Services;
 using Filee.Core.Platform;
 using Filee.Core.Settings;
@@ -302,8 +305,55 @@ public static AppBuilder BuildAvaloniaApp()
     }
 }
 
-/// <summary>Embedded Noto fonts: <c>fonts:Filee#Noto Sans</c>, <c>#Noto Sans JP</c>, <c>#Noto Sans KR</c>,
-/// <c>#Noto Sans SC</c>, <c>#Noto Sans Devanagari</c>, <c>#Noto Sans Arabic</c>, <c>#Noto Sans Thai</c>.</summary>
-internal sealed class FileeFontCollection() : Avalonia.Media.Fonts.EmbeddedFontCollection(
-    new Uri("fonts:Filee", UriKind.Absolute),
-    new Uri("avares://Filee/Assets/Fonts", UriKind.Absolute));
+/// <summary>
+/// Embedded Noto fonts: <c>fonts:Filee#Noto Sans</c>, <c>#Noto Sans JP</c>, <c>#Noto Sans KR</c>,
+/// <c>#Noto Sans SC</c>, <c>#Noto Sans Devanagari</c>, <c>#Noto Sans Arabic</c>, <c>#Noto Sans Thai</c>.
+/// <para>
+/// A letter is only matched in the family asked for, so the order of SoftPalette's font chain decides (#1095). Avalonia's
+/// own collection, asked for Noto Sans about a Han character, answers with the first family of its own that has it,
+/// Noto Sans JP, before the font manager reaches the chain's next family: every language drew Han in Japanese shapes,
+/// up to the first character that font lacks. Text has no culture to go by either, as TextBlock passes none. It takes
+/// this wrapper rather than a subclass: the font manager skips the virtual method on Avalonia's collections.
+/// </para>
+/// </summary>
+internal sealed class FileeFontCollection : IFontCollection
+{
+    private readonly EmbeddedFontCollection _fonts = new(
+        new Uri("fonts:Filee", UriKind.Absolute),
+        new Uri("avares://Filee/Assets/Fonts", UriKind.Absolute));
+
+    public Uri Key => _fonts.Key;
+    public int Count => _fonts.Count;
+    public FontFamily this[int index] => _fonts[index];
+
+    public bool TryMatchCharacter(int codepoint, FontStyle fontStyle, FontWeight fontWeight, FontStretch fontStretch,
+        string? familyName, CultureInfo? culture, out Typeface typeface)
+    {
+        typeface = default;
+        // No culture from here on: the family is settled, and one would send it looking elsewhere again.
+        return familyName is not null
+            && _fonts.TryGetGlyphTypeface(familyName, fontStyle, fontWeight, fontStretch, out var face)
+            && face.CharacterToGlyphMap.TryGetGlyph(codepoint, out _)
+            && _fonts.TryMatchCharacter(codepoint, fontStyle, fontWeight, fontStretch, familyName, null, out typeface);
+    }
+
+    public bool TryGetGlyphTypeface(string familyName, FontStyle style, FontWeight weight, FontStretch stretch,
+        [NotNullWhen(true)] out GlyphTypeface? glyphTypeface) =>
+        _fonts.TryGetGlyphTypeface(familyName, style, weight, stretch, out glyphTypeface);
+
+    public bool TryGetFamilyTypefaces(string familyName,
+        [NotNullWhen(true)] out IReadOnlyList<Typeface>? familyTypefaces) =>
+        _fonts.TryGetFamilyTypefaces(familyName, out familyTypefaces);
+
+    public bool TryCreateSyntheticGlyphTypeface(GlyphTypeface glyphTypeface, FontStyle style, FontWeight weight, FontStretch stretch,
+        [NotNullWhen(true)] out GlyphTypeface? syntheticGlyphTypeface) =>
+        _fonts.TryCreateSyntheticGlyphTypeface(glyphTypeface, style, weight, stretch, out syntheticGlyphTypeface);
+
+    public bool TryGetNearestMatch(string familyName, FontStyle style, FontWeight weight, FontStretch stretch,
+        [NotNullWhen(true)] out GlyphTypeface? glyphTypeface) =>
+        _fonts.TryGetNearestMatch(familyName, style, weight, stretch, out glyphTypeface);
+
+    public IEnumerator<FontFamily> GetEnumerator() => _fonts.GetEnumerator();
+    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();
+    public void Dispose() => ((IDisposable)_fonts).Dispose();
+}

src/Filee.App/Services/SoftPalette.cs

@@ -33,9 +33,10 @@ private static readonly (string Embedded, string System) Chinese =
 
     /// <summary>
     /// Traditional Chinese has no embedded font (two more Noto CJK files would add 11 MB): Windows and macOS always carry
-    /// these. Listed before the embedded Noto Sans SC, whose Han shapes differ, so zh-TW text takes Traditional shapes.
+    /// their two, Linux often has the Noto one. Listed before the embedded Noto Sans SC, whose Han shapes differ, so zh-TW
+    /// text takes Traditional shapes.
     /// </summary>
-    private const string TraditionalChineseSystem = "Microsoft JhengHei UI, Microsoft JhengHei, PingFang TC";
+    private const string TraditionalChineseSystem = "Microsoft JhengHei UI, Microsoft JhengHei, PingFang TC, Noto Sans CJK TC";
 
     /// <summary>All resource keys written by ApplyAccent that must be cleared from top level when per-variant dictionaries are used.</summary>
     public static readonly string[] AccentResourceKeys =

tests/Filee.App.Tests/LocalizationResourceTests.cs

@@ -2,6 +2,8 @@
 using Avalonia.Controls;
 using Avalonia.Headless.XUnit;
 using Avalonia.Media;
+using Avalonia.Media.TextFormatting;
+using Avalonia.Platform;
 using Avalonia.VisualTree;
 using Filee.App.Controls;
 using Filee.App.Services;
@@ -30,6 +32,77 @@ public void Arabic_font_chain_draws_trimming_ellipsis_and_matches_arabic_letters
         finally { TestServices.EnsureInitialized("en"); }
     }
 
+    /// <summary>What a piece of text is drawn in, run by run, as a TextBlock lays it out (it passes no culture).</summary>
+    private static List<(string Text, string Family)> FontsDrawing(string text, string language, FontWeight weight = FontWeight.Normal)
+    {
+        var family = FontFamily.Parse(ThemeService.FontFamilies(language));
+        using var layout = new TextLayout(text, new Typeface(family, FontStyle.Normal, weight), 14, Brushes.Black);
+        return [.. layout.TextLines.SelectMany(l => l.TextRuns).OfType<ShapedTextRun>()
+            .Select(r => (r.Text.ToString(), r.GlyphRun.GlyphTypeface.FamilyName))];
+    }
+
+    /// <summary>
+    /// #1095: these Han characters differ between the regions and every embedded CJK font has them, so only the font
+    /// chain's order can pick the font. Before, it was Noto Sans JP in every language.
+    /// </summary>
+    [AvaloniaTheory]
+    [InlineData("zh-CN", "Noto Sans SC")]
+    [InlineData("ja", "Noto Sans JP")]
+    [InlineData("ko", "Noto Sans KR")]
+    [InlineData("en", "Noto Sans KR")]
+    public void Han_characters_take_the_embedded_font_of_the_ui_languages_region(string language, string font)
+    {
+        Assert.SkipUnless(AssetLoader.Exists(new Uri("avares://Filee/Assets/Fonts/NotoSansSC-Regular.otf")), "Needs build/fetch-fonts.ps1.");
+        TestServices.EnsureInitialized(language);
+        try
+        {
+            var family = FontFamily.Parse(ThemeService.FontFamilies(language));
+            foreach (var character in "将系与所")
+            {
+                Assert.True(FontManager.Current.TryMatchCharacter(character, FontStyle.Normal, FontWeight.Normal, FontStretch.Normal, family, null, out var typeface));
+                Assert.Equal(font, typeface.GlyphTypeface.FamilyName);
+            }
+            foreach (var weight in new[] { FontWeight.Normal, FontWeight.SemiBold, FontWeight.Bold })
+                Assert.Equal([("Save ", "Noto Sans"), ("将系与所", font)],
+                    FontsDrawing("Save 将系与所", language, weight));
+        }
+        finally { TestServices.EnsureInitialized("en"); }
+    }
+
+    /// <summary>#1095's strings: 跟随系 stayed Japanese up to 统, the first character Noto Sans JP doesn't have.</summary>
+    [AvaloniaTheory]
+    [InlineData("将文件拖放到这里")]
+    [InlineData("跟随系统")]
+    [InlineData("与源文件相同的文件夹")]
+    [InlineData("所有转换都保留原文件日期")]
+    public void Simplified_chinese_text_is_one_run_of_noto_sans_sc(string text)
+    {
+        Assert.SkipUnless(AssetLoader.Exists(new Uri("avares://Filee/Assets/Fonts/NotoSansSC-Regular.otf")), "Needs build/fetch-fonts.ps1.");
+        TestServices.EnsureInitialized("zh-CN");
+        try
+        {
+            Assert.Equal([(text, "Noto Sans SC")], FontsDrawing(text, "zh-CN"));
+        }
+        finally { TestServices.EnsureInitialized("en"); }
+    }
+
+    /// <summary>
+    /// #1095: the only embedded font zh-TW may land on is Noto Sans SC, and that only on a machine with no Traditional
+    /// one (macOS answers PingFang TC here, Windows Microsoft JhengHei UI).
+    /// </summary>
+    [AvaloniaFact]
+    public void Traditional_chinese_text_reaches_the_system_traditional_font()
+    {
+        Assert.SkipUnless(AssetLoader.Exists(new Uri("avares://Filee/Assets/Fonts/NotoSansSC-Regular.otf")), "Needs build/fetch-fonts.ps1.");
+        TestServices.EnsureInitialized("zh-TW");
+        try
+        {
+            string[] allowed = ["Microsoft JhengHei UI", "Microsoft JhengHei", "PingFang TC", "Noto Sans CJK TC", "Noto Sans SC"];
+            Assert.All(FontsDrawing("將文件體", "zh-TW"), run => Assert.Contains(run.Family, allowed));
+        }
+        finally { TestServices.EnsureInitialized("en"); }
+    }
+
     [AvaloniaFact]
     public void Arabic_mirrors_the_main_window_and_english_brings_it_back()
     {