Ignore weak HElem at the beginning and end of an LR group (#2950)

crates/typst/src/math/fragment.rs

@@ -18,7 +18,7 @@ pub enum MathFragment {
     Glyph(GlyphFragment),
     Variant(VariantFragment),
     Frame(FrameFragment),
-    Spacing(Abs),
+    Spacing(SpacingFragment),
     Space(Abs),
     Linebreak,
     Align,
@@ -34,7 +34,7 @@ impl MathFragment {
             Self::Glyph(glyph) => glyph.width,
             Self::Variant(variant) => variant.frame.width(),
             Self::Frame(fragment) => fragment.frame.width(),
-            Self::Spacing(amount) => *amount,
+            Self::Spacing(spacing) => spacing.width,
             Self::Space(amount) => *amount,
             _ => Abs::zero(),
         }
@@ -205,6 +205,12 @@ impl From<FrameFragment> for MathFragment {
     }
 }
 
+impl From<SpacingFragment> for MathFragment {
+    fn from(fragment: SpacingFragment) -> Self {
+        Self::Spacing(fragment)
+    }
+}
+
 #[derive(Clone)]
 pub struct GlyphFragment {
     pub id: GlyphId,
@@ -467,6 +473,12 @@ impl FrameFragment {
     }
 }
 
+#[derive(Debug, Clone)]
+pub struct SpacingFragment {
+    pub width: Abs,
+    pub weak: bool,
+}
+
 /// Look up the italics correction for a glyph.
 fn italics_correction(ctx: &MathContext, id: GlyphId) -> Option<Abs> {
     Some(ctx.table.glyph_info?.italic_corrections?.get(id)?.scaled(ctx))

crates/typst/src/math/lr.rs

@@ -3,7 +3,9 @@ use unicode_math_class::MathClass;
 use crate::diag::SourceResult;
 use crate::foundations::{elem, func, Content, NativeElement, Resolve, Smart};
 use crate::layout::{Abs, Em, Length, Rel};
-use crate::math::{GlyphFragment, LayoutMath, MathContext, MathFragment, Scaled};
+use crate::math::{
+    GlyphFragment, LayoutMath, MathContext, MathFragment, Scaled, SpacingFragment,
+};
 use crate::text::TextElem;
 
 /// How much less high scaled delimiters can be than what they wrap.
@@ -77,6 +79,19 @@ impl LayoutMath for LrElem {
             }
         }
 
+        // Remove weak SpacingFragment immediately after the opening or immediately
+        // before the closing.
+        let original_len = fragments.len();
+        let mut index = 0;
+        fragments.retain(|fragment| {
+            index += 1;
+            (index != 2 && index + 1 != original_len)
+                || !matches!(
+                    fragment,
+                    MathFragment::Spacing(SpacingFragment { weak: true, .. })
+                )
+        });
+
         ctx.extend(fragments);
 
         Ok(())

crates/typst/src/math/mod.rs

@@ -279,7 +279,10 @@ impl LayoutMath for Content {
         if let Some(elem) = self.to::<HElem>() {
             if let Spacing::Rel(rel) = elem.amount() {
                 if rel.rel.is_zero() {
-                    ctx.push(MathFragment::Spacing(rel.abs.resolve(ctx.styles())));
+                    ctx.push(SpacingFragment {
+                        width: rel.abs.resolve(ctx.styles()),
+                        weak: elem.weak(ctx.styles()),
+                    });
                 }
             }
             return Ok(());

crates/typst/src/math/row.rs

@@ -10,6 +10,8 @@ use crate::math::{
 };
 use crate::model::ParElem;
 
+use super::fragment::SpacingFragment;
+
 pub const TIGHT_LEADING: Em = Em::new(0.25);
 
 #[derive(Debug, Default, Clone)]
@@ -279,8 +281,9 @@ impl MathRow {
         while let Some(fragment) = iter.next() {
             if space_is_visible {
                 match fragment {
-                    MathFragment::Space(s) | MathFragment::Spacing(s) => {
-                        items.push(MathParItem::Space(s));
+                    MathFragment::Space(width)
+                    | MathFragment::Spacing(SpacingFragment { width, .. }) => {
+                        items.push(MathParItem::Space(width));
                         continue;
                     }
                     _ => {}

crates/typst/src/math/spacing.rs

@@ -2,7 +2,7 @@ use unicode_math_class::MathClass;
 
 use crate::foundations::{NativeElement, Scope};
 use crate::layout::{Abs, Em, HElem};
-use crate::math::{MathFragment, MathSize};
+use crate::math::{MathFragment, MathSize, SpacingFragment};
 
 pub(super) const THIN: Em = Em::new(1.0 / 6.0);
 pub(super) const MEDIUM: Em = Em::new(2.0 / 9.0);
@@ -28,8 +28,9 @@ pub(super) fn spacing(
     use MathClass::*;
 
     let class = |f: &MathFragment| f.class().unwrap_or(Special);
-    let resolve = |v: Em, f: &MathFragment| {
-        Some(MathFragment::Spacing(f.font_size().map_or(Abs::zero(), |size| v.at(size))))
+    let resolve = |v: Em, size_ref: &MathFragment| -> Option<MathFragment> {
+        let width = size_ref.font_size().map_or(Abs::zero(), |size| v.at(size));
+        Some(SpacingFragment { width, weak: false }.into())
     };
     let script =
         |f: &MathFragment| f.style().map_or(false, |s| s.size <= MathSize::Script);

tests/typ/math/delimited.typ

@@ -51,3 +51,9 @@ $ { 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) } $
+
+---
+// Test ignoring weak spacing immediately after the opening
+// and immediately before the closing.
+
+$ [#h(1em, weak: true)A(dif x, f(x) dif x)sum#h(1em, weak: true)] $