Fix weak spacing broken in math (#1966)

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

@@ -418,11 +418,12 @@ impl LayoutMath for Content {
             return realized.layout_math(ctx);
         }
 
-        if let Some(children) = self.to_sequence() {
+        if self.is_sequence() {
             let mut bb = BehavedBuilder::new();
-            for child in children {
-                bb.push(child.clone(), StyleChain::default());
-            }
+            self.sequence_recursive_for_each(&mut |child: &Content| {
+                bb.push(child.clone(), StyleChain::default())
+            });
+
             for (child, _) in bb.finish().0.iter() {
                 child.layout_math(ctx)?;
             }

crates/typst/src/model/content.rs

@@ -85,12 +85,25 @@ impl Content {
 
     /// Access the children if this is a sequence.
     pub fn to_sequence(&self) -> Option<impl Iterator<Item = &Self>> {
-        if !self.is::<SequenceElem>() {
+        if !self.is_sequence() {
             return None;
         }
         Some(self.attrs.iter().filter_map(Attr::child))
     }
 
+    pub fn is_sequence(&self) -> bool {
+        self.is::<SequenceElem>()
+    }
+
+    /// Also auto expands sequence of sequences into flat sequence
+    pub fn sequence_recursive_for_each(&self, f: &mut impl FnMut(&Self)) {
+        if let Some(childs) = self.to_sequence() {
+            childs.for_each(|c| c.sequence_recursive_for_each(f));
+        } else {
+            f(self);
+        }
+    }
+
     /// Access the child and styles.
     pub fn to_styled(&self) -> Option<(&Content, &Styles)> {
         if !self.is::<StyledElem>() {

tests/typ/math/spacing.typ

@@ -39,3 +39,10 @@ $a cancel(equiv) b overline(+) c arrow(-) d hat(=>) e cancel(log) 5 dot(op("ln")
 $a overbrace(equiv) b underline(+) c grave(-) d underbracket(=>) e circle(log) 5 caron(op("ln")) 6$ \
 \
 $a attach(equiv, tl: a, tr: b) b attach(limits(+), t: a, b: b) c tilde(-) d breve(=>) e attach(limits(log), t: a, b: b) 5 attach(op("ln"), tr: a, bl: b) 6$
+---
+// Test weak spacing
+$integral f(x) dif x$,
+// Not weak
+$integral f(x) thin dif x$,
+// Both are weak, collide
+$integral f(x) #h(0.166em, weak: true)dif x$