Implement mid() for middle delimiters in lr() (#2760)

crates/typst/src/math/fragment.rs

@@ -297,6 +297,7 @@ impl GlyphFragment {
             span: self.span,
             limits: self.limits,
             frame: self.into_frame(),
+            mid_stretched: None,
         }
     }
 
@@ -360,6 +361,7 @@ pub struct VariantFragment {
     pub class: Option<MathClass>,
     pub span: Span,
     pub limits: Limits,
+    pub mid_stretched: Option<bool>,
 }
 
 impl VariantFragment {

crates/typst/src/math/lr.rs

@@ -57,6 +57,7 @@ impl LayoutMath for LrElem {
             .resolve(ctx.styles())
             .relative_to(2.0 * max_extent);
 
+        // Scale up fragments at both ends.
         match fragments.as_mut_slice() {
             [one] => scale(ctx, one, height, None),
             [first, .., last] => {
@@ -66,12 +67,58 @@ impl LayoutMath for LrElem {
             _ => {}
         }
 
+        // Handle MathFragment::Variant fragments that should be scaled up.
+        for fragment in &mut fragments {
+            if let MathFragment::Variant(ref mut variant) = fragment {
+                if variant.mid_stretched == Some(false) {
+                    variant.mid_stretched = Some(true);
+                    scale(ctx, fragment, height, Some(MathClass::Large));
+                }
+            }
+        }
+
         ctx.extend(fragments);
 
         Ok(())
     }
 }
 
+/// Scales contents vertically to the nearest surrounding lr() group.
+///
+/// ```example
+/// $ { x mid(|) sum_(i=1)^oo phi_i (x) < 1 } $
+/// ```
+#[elem(LayoutMath)]
+pub struct MidElem {
+    /// The content to be scaled.
+    #[required]
+    pub body: Content,
+}
+
+impl LayoutMath for MidElem {
+    #[tracing::instrument(skip(ctx))]
+    fn layout_math(&self, ctx: &mut MathContext) -> SourceResult<()> {
+        let mut fragments = ctx.layout_fragments(self.body())?;
+
+        for fragment in &mut fragments {
+            match fragment {
+                MathFragment::Glyph(glyph) => {
+                    let mut new = glyph.clone().into_variant();
+                    new.mid_stretched = Some(false);
+                    *fragment = MathFragment::Variant(new);
+                }
+                MathFragment::Variant(variant) => {
+                    variant.mid_stretched = Some(false);
+                }
+                _ => {}
+            }
+        }
+
+        ctx.extend(fragments);
+        Ok(())
+    }
+}
+
 /// Scale a math fragment to a height.
 fn scale(
     ctx: &mut MathContext,

crates/typst/src/math/mod.rs

@@ -161,6 +161,7 @@ pub fn module() -> Module {
     math.define_elem::<EquationElem>();
     math.define_elem::<TextElem>();
     math.define_elem::<LrElem>();
+    math.define_elem::<MidElem>();
     math.define_elem::<AttachElem>();
     math.define_elem::<ScriptsElem>();
     math.define_elem::<LimitsElem>();

crates/typst/src/math/stretch.rs

@@ -187,6 +187,7 @@ fn assemble(
         class: base.class,
         span: base.span,
         limits: base.limits,
+        mid_stretched: None,
     }
 }
 

tests/typ/math/delimited.typ

@@ -43,3 +43,10 @@ $ lr(
     text("(", fill: #green) a/b
     text(")", fill: #blue)
   ) $
+
+---
+// Test middle functions
+$ { x mid(|) sum_(i=1)^oo phi_i (x) < 1 } \
+  { integral |x| dif x
+      mid(bar.v.double)
+    floor(hat(A) mid(|) { x mid(|) y } mid(|) A) } $