diff --git a/crates/doc/src/tests.rs b/crates/doc/src/tests.rs
index 928dca55..41e233c4 100644
--- a/crates/doc/src/tests.rs
+++ b/crates/doc/src/tests.rs
@@ -560,6 +560,16 @@ fn is_ancestor_holds_for_very_deep_chains() {
     assert!(matches!(d.reparent(root, leaf, None), Err(DocError::Cycle(..))));
 }
 
+#[test]
+fn text_insert_at_run_boundary_keeps_runs_disjoint() {
+    let mut t = TextData::new("0123456789");
+    t.restyle(0, 5, |s| s.font_size = 20.0);
+    t.restyle(5, 10, |s| s.font_size = 30.0);
+    t.replace_range(5, 5, "xx");
+    let runs: Vec<(usize, usize)> = t.runs.iter().map(|r| (r.start, r.end)).collect();
+    assert_eq!(runs, vec![(0, 7), (7, 12)]);
+}
+
 #[test]
 fn many_small_vector_subpaths_build_and_stroke_without_rescans() {
     // Many closed subpaths whose last point repeats the start (#F-332), a long open chain (#F-334)
diff --git a/crates/doc/src/text.rs b/crates/doc/src/text.rs
index 458426ad..d2abfed9 100644
--- a/crates/doc/src/text.rs
+++ b/crates/doc/src/text.rs
@@ -579,7 +579,14 @@ impl TextData {
         }
         self.runs.sort_by_key(|r| r.start);
         let mut merged: Vec<StyleRun> = Vec::with_capacity(self.runs.len());
-        for r in std::mem::take(&mut self.runs) {
+        for mut r in std::mem::take(&mut self.runs) {
+            // Runs never overlap: a later run starts where the previous one ends at the earliest.
+            if let Some(prev) = merged.last() {
+                r.start = r.start.max(prev.end);
+                if r.start >= r.end {
+                    continue;
+                }
+            }
             match merged.last_mut() {
                 Some(prev) if prev.end == r.start && prev.style == r.style => prev.end = r.end,
                 _ => merged.push(r),