gfx: Change the mapping from Mac weights to CSS weights so that 0.0 maps

to 400, not 500.

CSS `normal` font-weight is specified as 400, while Mac "Regular" font
weight is reported as 0.0. On the Mac, we need to center the two ranges
on the same value to avoid choosing "Light" fonts where "Regular" would
have been more appropriate.

Closes #9487.

fix for mac

components/gfx/platform/macos/font.rs

@@ -103,16 +103,20 @@ impl FontHandleMethods for FontHandle {
 
     fn boldness(&self) -> font_weight::T {
         let normalized = self.ctfont.all_traits().normalized_weight();  // [-1.0, 1.0]
-        let normalized = (normalized + 1.0) / 2.0 * 9.0;  // [0.0, 9.0]
-        match normalized {
-            v if v < 1.0 => font_weight::T::Weight100,
-            v if v < 2.0 => font_weight::T::Weight200,
-            v if v < 3.0 => font_weight::T::Weight300,
-            v if v < 4.0 => font_weight::T::Weight400,
-            v if v < 5.0 => font_weight::T::Weight500,
-            v if v < 6.0 => font_weight::T::Weight600,
-            v if v < 7.0 => font_weight::T::Weight700,
-            v if v < 8.0 => font_weight::T::Weight800,
+        let normalized = if normalized <= 0.0 {
+            4.0 + normalized * 3.0  // [1.0, 4.0]
+        } else {
+            4.0 + normalized * 5.0  // [4.0, 9.0]
+        }; // [1.0, 9.0], centered on 4.0
+        match normalized.round() as u32 {
+            1 => font_weight::T::Weight100,
+            2 => font_weight::T::Weight200,
+            3 => font_weight::T::Weight300,
+            4 => font_weight::T::Weight400,
+            5 => font_weight::T::Weight500,
+            6 => font_weight::T::Weight600,
+            7 => font_weight::T::Weight700,
+            8 => font_weight::T::Weight800,
             _ => font_weight::T::Weight900,
         }
     }