LT-22854: Control the Avalonia interface font from the theme

.claude/skills/fieldworks-avalonia-ui/references/style-system.md

@@ -8,7 +8,7 @@ toward the Fluent theme per the migration hub's `architecture-patterns.md` §12.
 
 ## Why a global token system (and why it renders headlessly)
 
-`FwAvaloniaApp` adds `new FluentTheme()`, which ships ~14px fonts and ~32px control floors app-wide.
+`FwAvaloniaApp` adds Semi core plus Ursa Semi (no Fluent), which ships ~14px fonts and ~32px control floors app-wide.
 Density/borders must be layered ON TOP of Fluent. The reliable mechanism — proven by a prior agent —
 is **per-control-tree `.Styles`**, added to each view's own `Styles` collection by its view ctor.
 App-level `Application.Styles` did **not** apply in the headless test app, so that path is rejected.
@@ -42,6 +42,11 @@ sit inside a `Border.fwFieldHost` that supplies the box.
 directly (not three independently-maintained copies) -- see
 `Src/Common/FwAvalonia/FwThemeResources.cs`.
 
+**Font family:** Segoe UI, named by `FwUiFontFamily`. `FwUiFont.ApplyTo` registers
+bundled Andika and points Semi's font resources at the theme family stack. Named
+UI-language fallbacks and the platform default follow it. Data text keeps its
+writing-system font. Measure UI text with `FwUiFont.UiFontFamily`. See ADR 0004.
+
 **Control height:** `TextBox`/`ComboBox`/`Button` `MinHeight = 24` (WinForms runs ~21-23px; 24 is the
 pointer-accessibility floor — see the "Why `DialogMinControlHeight` is 24, not 22" note below — still far
 from Fluent's ~32px).

Build/Src/FwBuildTasks/GenerateTokenKeys.cs

@@ -38,9 +38,10 @@ public class GenerateTokenKeys : Task
 	{
 		private static readonly XNamespace XamlNs = "http://schemas.microsoft.com/winfx/2006/xaml";
 
-		/// <summary>Every top-level ThemeDictionary entry becomes a key constant.</summary>
+		/// <summary>Every top-level ThemeDictionary entry becomes a key constant. FontFamily
+		/// entries are included, so the interface font family has a constant too.</summary>
 		private static readonly HashSet<string> TokenElementNames = new HashSet<string>(
-			new[] { "SolidColorBrush", "Color", "Double", "Thickness", "CornerRadius", "StaticResource" });
+			new[] { "SolidColorBrush", "Color", "Double", "Thickness", "CornerRadius", "FontFamily", "StaticResource" });
 
 		/// <summary>.axaml files whose EVERY x:Key becomes a generated constant.</summary>
 		[Required]

Docs/architecture/adr/0004-interface-font-controlled-by-the-theme.md

@@ -0,0 +1,22 @@
+# The Avalonia interface font is controlled by the theme
+
+LT-22854. The `FwUiFontFamily` token names Segoe UI as the default interface
+font on Windows. Segoe UI preserves the density of the current interface; the
+proposed Andika default has taller line metrics. Line-height tokens remain a
+separate decision.
+
+`FwUiFont.ApplyTo` registers the bundled Andika collection and assigns the
+interface font stack to Semi's `DefaultFontFamily`,
+`ContentControlThemeFontFamily`, and `SemiFontFamilyRegular` resources. The
+stack combines the theme default, fallbacks for the UI language, and the
+platform default. It is chosen at startup and applies across the application:
+menus and other interface chrome should not change when the project changes.
+
+The language project's `UiElement` style continues to control UI-writing-system
+text drawn in Views; Avalonia interface chrome does not read that style. Data
+text continues to use its writing-system font. Andika 7.000 remains bundled
+in a private collection; a plain `Andika` family still names the system font
+for writing-system data.
+
+Windows validation must measure density, clipping and script glyph fallback.
+Screenshot baselines still depict Andika and must be refreshed on Windows.

Docs/architecture/adr/README.md

@@ -28,3 +28,4 @@ editing the next time the code changes, it belongs in one of the other three.
 | [0001](0001-alias-semi-semantic-tokens.md) | Alias Semi's semantic tokens instead of inventing a FieldWorks palette |
 | [0002](0002-whole-tree-token-hygiene-check.md) | token-hygiene.ps1 enforces full conformance across the Avalonia surface, with no grandfathering |
 | [0003](0003-geometric-assertions-not-pixel-diff.md) | Assert geometry, not pixel diffs, for layout parity |
+| [0004](0004-interface-font-controlled-by-the-theme.md) | Control the Avalonia interface font through a theme token; data text keeps its writing-system font |

Docs/migration/adjust-the-layout.md

@@ -173,8 +173,9 @@ host. Treat difficulty here as a design signal, not a tooling problem.
 - **The host window is WinForms, not Avalonia.** Title bar, icon, and close
   button belong to the modal host form. You cannot style them from the
   `.axaml`, and neither loop above shows them.
-- **Converted dialogs render on the Fluent background, not WinForms grey**, and
-  use the Fluent font family. That is stack-wide, not something your dialog did.
+- **Converted dialogs render on the Semi theme background, not WinForms grey**,
+  and use the FieldWorks interface font (`FwUiFontFamily`,
+  ADR 0004). That is stack-wide, not something your dialog did.
   Changing it means editing `DialogTheme.axaml`'s `fwDialogRoot` style, which
   restyles **every** converted dialog -- treat it as its own change, with dark
   mode considered.

Src/Common/FwAvalonia/Detail/FwFieldControls.cs

@@ -94,7 +94,7 @@ public static double ComputeWsAbbrevColumnWidth(DetailModel model)
 			if (model == null)
 				return FwAvaloniaDensity.WsAbbrevWidth;
 
-			var typeface = new Typeface(FontFamily.Default);
+			var typeface = new Typeface(FwUiFont.UiFontFamily);
 			double widest = 0;
 			foreach (var abbrev in model.Fields.Where(f => f.IsMultiStringRow)
 				.SelectMany(f => f.Values).Select(v => v.WsAbbrev)

Src/Common/FwAvalonia/FwAvaloniaApp.cs

@@ -36,6 +36,8 @@ public override void Initialize()
 			// Shared dialog spacing/border tokens, so the Dialog* keys this assembly
 			// publishes as generated constants are actually registered here too.
 			MergeTokens("avares://FwAvaloniaTheme/Tokens/DialogTokens.axaml");
+			// After the token merge: the interface font family is itself a token.
+			FwUiFont.ApplyTo(this);
 		}
 
 		private void MergeTokens(string avaresUri)

Src/Common/FwAvalonia/FwAvaloniaTests/AvaloniaDialogHostTests.cs

@@ -46,7 +46,7 @@ private sealed class DisposableControl : Control, IDisposable
 			public void Dispose() => DisposeCount++;
 		}
 
-		[Test]
+		[AvaloniaTest]
 		public void DisposeDialogResources_DisposesDisposableBodyAndViewModel()
 		{
 			var body = new DisposableControl();
@@ -58,7 +58,7 @@ public void DisposeDialogResources_DisposesDisposableBodyAndViewModel()
 			Assert.That(vm.DisposeCount, Is.EqualTo(1), "the VM is disposed if IDisposable (it owns its resources)");
 		}
 
-		[Test]
+		[AvaloniaTest]
 		public void DisposeDialogResources_NonDisposableViewModel_DoesNotThrow()
 		{
 			Assert.DoesNotThrow(() => AvaloniaDialogHost.DisposeDialogResources(new Control(), new PlainVm()));

Src/Common/FwAvalonia/FwAvaloniaTests/DetailPreviewTests.cs

@@ -58,6 +58,18 @@ public void Preview_WsText_UsesConfiguredFonts()
 			Assert.That(Find<TextBox>(window, "LexemeFormEditor.en").FontFamily.Name, Is.EqualTo("Times New Roman"));
 		}
 
+		[AvaloniaTest]
+		public void Preview_WsText_KeepsWsFonts_WhileLabelsUseTheInterfaceFont()
+		{
+			var window = ShowPreview();
+			Assert.That(Find<TextBox>(window, "LexemeFormEditor.seh").FontFamily.Name, Is.EqualTo("Charis SIL"));
+			Assert.That(Find<TextBox>(window, "LexemeFormEditor.en").FontFamily.Name, Is.EqualTo("Times New Roman"));
+
+			var label = window.GetVisualDescendants().OfType<TextBlock>()
+				.First(t => !string.IsNullOrEmpty(t.Text) && t.FindAncestorOfType<TextBox>() == null);
+			Assert.That(label.FontFamily, Is.EqualTo(SIL.FieldWorks.Common.FwAvalonia.FwUiFont.UiFontFamily));
+		}
+
 		[AvaloniaTest]
 		public void Preview_MorphTypeChooser_UsesSharedOptionPicker_AndReturnsFocus()
 		{