Add more documentation

crates/usvg/src/text/layout.rs

@@ -95,17 +95,29 @@ impl PositionedGlyph {
         pixels_per_em: f32,
         height: f32,
     ) -> Transform {
+        // In contrast to CBDT, we also need to look at the outline bbox of the glyph and add a shift if necessary.
+        let bbox_x_shift = self.font_size * (-x_min / self.units_per_em as f32);
+
+        let bbox_y_shift = if y_min.approx_zero_ulps(4) {
+            // For unknown reasons, using Apple Color Emoji will lead to a vertical shift on MacOS, but this shift
+            // doesn't seem to be coming from the font and most likely is somehow hardcoded. On Windows,
+            // this shift will not be applied. However, if this shift is not applied the emojis are a bit
+            // too high up when being together with other text, so we try to imitate this.
+            // See also https://github.com/harfbuzz/harfbuzz/issues/2679#issuecomment-1345595425
+            // So whenever the y-shift is 0, we approximate this vertical shift that seems to be produced by it.
+            // This value seems to be pretty close to what is happening on MacOS.
+            // We can still remove this if it turns out to be a problem, but Apple Color Emoji is pretty
+            // much the only `sbix` font out there and they all seem to have a y-shift of 0, so it
+            // makes sense to keep it.
+            0.128 * self.font_size
+        } else {
+            self.font_size * (-y_min / self.units_per_em as f32)
+        };
+
         self.span_ts
             .pre_concat(self.cluster_ts)
             .pre_concat(self.glyph_ts)
-            // In contrast to CBDT, we also need to look at the outline bbox of the glyph and add a shift if necessary.
-            // For unknown reasons, using Apple Color Emoji will lead to a vertical shift on MacOS, but this shift
-            // doesn't seem to be coming from the font and most likely is somehow hardcoded into CoreText. But we
-            // ignore this. See also https://github.com/harfbuzz/harfbuzz/issues/2679#issuecomment-1345595425
-            .pre_concat(Transform::from_translate(
-                self.font_size * (-x_min / self.units_per_em as f32),
-                self.font_size * (-y_min / self.units_per_em as f32),
-            ))
+            .pre_concat(Transform::from_translate(bbox_x_shift, bbox_y_shift))
             .pre_concat(Transform::from_scale(
                 self.font_size / pixels_per_em,
                 self.font_size / pixels_per_em,