diff --git a/crates/typst-layout/src/math/lr.rs b/crates/typst-layout/src/math/lr.rs
index 19176ee88..bf8235411 100644
--- a/crates/typst-layout/src/math/lr.rs
+++ b/crates/typst-layout/src/math/lr.rs
@@ -2,6 +2,7 @@ use typst_library::diag::SourceResult;
 use typst_library::foundations::{Packed, StyleChain};
 use typst_library::layout::{Abs, Axis, Rel};
 use typst_library::math::{EquationElem, LrElem, MidElem};
+use typst_utils::SliceExt;
 use unicode_math_class::MathClass;
 
 use super::{stretch_fragment, MathContext, MathFragment, DELIM_SHORT_FALL};
@@ -29,15 +30,7 @@ pub fn layout_lr(
     let mut fragments = ctx.layout_into_fragments(body, styles)?;
 
     // Ignore leading and trailing ignorant fragments.
-    let start_idx = fragments
-        .iter()
-        .position(|f| !f.is_ignorant())
-        .unwrap_or(fragments.len());
-    let end_idx = fragments
-        .iter()
-        .skip(start_idx)
-        .rposition(|f| !f.is_ignorant())
-        .map_or(start_idx, |i| start_idx + i + 1);
+    let (start_idx, end_idx) = fragments.split_prefix_suffix(|f| f.is_ignorant());
     let inner_fragments = &mut fragments[start_idx..end_idx];
 
     let axis = scaled!(ctx, styles, axis_height);
diff --git a/crates/typst-utils/src/lib.rs b/crates/typst-utils/src/lib.rs
index f3fe79d2c..b59fe2f73 100644
--- a/crates/typst-utils/src/lib.rs
+++ b/crates/typst-utils/src/lib.rs
@@ -128,6 +128,20 @@ pub trait SliceExt<T> {
     where
         F: FnMut(&T) -> K,
         K: PartialEq;
+
+    /// Computes two indices which split a slice into three parts.
+    ///
+    /// - A prefix which matches `f`
+    /// - An inner portion
+    /// - A suffix which matches `f` and does not overlap with the prefix
+    ///
+    /// If all elements match `f`, the prefix becomes `self` and the suffix
+    /// will be empty.
+    ///
+    /// Returns the indices at which the inner portion and the suffix start.
+    fn split_prefix_suffix<F>(&self, f: F) -> (usize, usize)
+    where
+        F: FnMut(&T) -> bool;
 }
 
 impl<T> SliceExt<T> for [T] {
@@ -157,6 +171,19 @@ impl<T> SliceExt<T> for [T] {
     fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F> {
         GroupByKey { slice: self, f }
     }
+
+    fn split_prefix_suffix<F>(&self, mut f: F) -> (usize, usize)
+    where
+        F: FnMut(&T) -> bool,
+    {
+        let start = self.iter().position(|v| !f(v)).unwrap_or(self.len());
+        let end = self
+            .iter()
+            .skip(start)
+            .rposition(|v| !f(v))
+            .map_or(start, |i| start + i + 1);
+        (start, end)
+    }
 }
 
 /// This struct is created by [`SliceExt::group_by_key`].