FontSize helper follows desktop text scale

AGENTS.md

@@ -32,6 +32,7 @@ testdata/snapshots/  <os>/<name>-<light|dark>.png
 
 - Signature: `func Name(c *ui.Context, value *T, opts NameOptions) *ui.Element`. Composites may return a named struct.
 - Zero `Options` gives the default look. No `map[string]any` config.
+- Explicit font sizes: `FontSize(c, theme.CaptionSize)`; it applies the desktop text scale.
 - Colors: `k := mujica.Tokens(c)`. Metrics: `theme.*` consts and `Density(c).Unit()` / `.ControlHeight()`.
 - Build on MyGo Bases (`ui.ButtonBase`, `ui.CheckboxBase`, `ui.SelectBase`, ...) or MyGo widgets restyled through the theme `Use` sets. Never reach into MyGo internals.
 - Focus: `internal.Ring(e, k.Focus, radius)` for the 2 DIP ring with 2 DIP gap.

theme.go

@@ -82,10 +82,7 @@ func Use(c *ui.Context, s Settings) {
 	if p.HighContrast {
 		k = k.HighContrast()
 	}
-	size := theme.BodySize
-	if p.TextScale > 0 {
-		size *= p.TextScale
-	}
+	size := FontSize(c, theme.BodySize)
 	c.SetTheme(&ui.Theme{
 		Dark:           k.Dark,
 		Background:     k.Background,
@@ -134,3 +131,11 @@ func Motion(c *ui.Context, d time.Duration) time.Duration {
 	}
 	return d
 }
+
+// FontSize scales a design size by the desktop's text scale.
+func FontSize(c *ui.Context, size float32) float32 {
+	if s := c.Preferences().TextScale; s > 0 {
+		return size * s
+	}
+	return size
+}

theme_test.go

@@ -41,3 +41,15 @@ func TestTokensWithoutUsePanics(t *testing.T) {
 	}()
 	ui.NewTester(func(c *ui.Context) { Tokens(c) }, 100, 100)
 }
+
+func TestFontSizeFollowsTextScale(t *testing.T) {
+	var got float32
+	tt := ui.NewTester(func(c *ui.Context) {
+		Use(c, Settings{})
+		got = FontSize(c, theme.CaptionSize)
+	}, 100, 100)
+	tt.SetPreferences(ui.Preferences{TextScale: 2})
+	if got != 2*theme.CaptionSize {
+		t.Fatalf("FontSize = %v, want %v", got, 2*theme.CaptionSize)
+	}
+}