Keep 8-bit alpha when printing colours with Color::to_css

Found while testing crates/color/src/lib.rs:198 (Color::to_css)

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/color/src/lib.rs

@@ -195,7 +195,14 @@ impl Color {
     /// CSS `rgba(...)` / hex text for copy-as-CSS.
     pub fn to_css(self) -> String {
         let [r, g, b, _] = self.to_rgba8();
-        if self.a >= 0.999 { self.to_css_hex() } else { format!("rgba({r}, {g}, {b}, {:.2})", self.a) }
+        if self.a >= 0.999 {
+            return self.to_css_hex();
+        }
+        // Two decimals when they keep the 8-bit alpha, three otherwise (near-transparent colours).
+        let a8 = |a: f32| (a.clamp(0.0, 1.0) * 255.0).round() as u8;
+        let short = format!("{:.2}", self.a);
+        let alpha = if short.parse::<f32>().is_ok_and(|v| a8(v) == a8(self.a)) { short } else { format!("{:.3}", self.a) };
+        format!("rgba({r}, {g}, {b}, {alpha})")
     }
 
     /// Linear interpolation in sRGB (as gradients render).

crates/color/src/tests.rs

@@ -133,3 +133,11 @@ fn malformed_alpha_or_extra_args_rejected() {
     assert_eq!(Color::parse_css("rgb(1,2,3)").unwrap().a, 1.0);
     assert_eq!(Color::parse_css("rgba(1,2,3,0.5)").unwrap().a, 0.5);
 }
+
+#[test]
+fn to_css_round_trips_every_alpha() {
+    for a in 0..=255u8 {
+        let c = Color::from_rgba8(10, 20, 30, a);
+        assert_eq!(Color::parse_css(&c.to_css()).unwrap().to_rgba8(), c.to_rgba8(), "alpha {a}: {}", c.to_css());
+    }
+}