Add support for weak elements in math (#1633)

crates/typst-library/src/math/mod.rs

@@ -47,6 +47,7 @@ use crate::meta::{
     Count, Counter, CounterUpdate, LocalName, Numbering, Outlinable, Refable,
 };
 use crate::prelude::*;
+use crate::shared::BehavedBuilder;
 use crate::text::{
     families, variant, FontFamily, FontList, LinebreakElem, SpaceElem, TextElem, TextSize,
 };
@@ -414,7 +415,11 @@ impl LayoutMath for Content {
         }
 
         if let Some(children) = self.to_sequence() {
+            let mut bb = BehavedBuilder::new();
             for child in children {
+                bb.push(child.clone(), StyleChain::default());
+            }
+            for (child, _) in bb.finish().0.iter() {
                 child.layout_math(ctx)?;
             }
             return Ok(());

crates/typst-library/src/math/op.rs

@@ -60,7 +60,7 @@ macro_rules! ops {
             );)*
 
             let dif = |d| {
-                HElem::new(THIN.into()).pack()
+                HElem::new(THIN.into()).with_weak(true).pack()
                     + MathStyleElem::new(TextElem::packed(d)).with_italic(Some(false)).pack()
             };
             math.define("dif", dif('d'));

tests/typ/math/frac.typ

@@ -20,6 +20,11 @@ $ binom(circle, square) $
 // Error: 8-13 missing argument: lower
 $ binom(x^2) $
 
+---
+// Test dif.
+$ (dif y)/(dif x), dif/x, x/dif, dif/dif \
+  frac(dif y, dif x), frac(dif, x), frac(x, dif), frac(dif, dif) $
+
 ---
 // Test associativity.
 $ 1/2/3 = (1/2)/3 = 1/(2/3) $