diff --git a/crates/engine/src/cmd/buildcmds.rs b/crates/engine/src/cmd/buildcmds.rs
index a4f620e9b..3f7da1629 100644
--- a/crates/engine/src/cmd/buildcmds.rs
+++ b/crates/engine/src/cmd/buildcmds.rs
@@ -354,7 +354,7 @@ fn sb_merge(s: &mut Session, p: &Value) -> Result<Value> {
         for (i, (sh, leaf)) in map.shapes.iter().zip(&leaves).enumerate() {
             let pieces: Vec<usize> = map.lines.iter().enumerate().filter(|(_, (l, _))| l.key == i as u64).map(|(k, _)| k).collect();
             if !pieces.is_empty() {
-                // An open path loses the pieces deleted and those between merged (or deleted) regions.
+                // An open path loses the pieces deleted and those between two merged (or deleted) regions.
                 let keep: Vec<usize> =
                     pieces.iter().copied().filter(|k| !touched.lines.contains(k) && !map.line_between(*k, &touched.regions)).collect();
                 if keep.len() == pieces.len() {
diff --git a/crates/engine/src/tests_build.rs b/crates/engine/src/tests_build.rs
index da2215a90..801fb9e6f 100644
--- a/crates/engine/src/tests_build.rs
+++ b/crates/engine/src/tests_build.rs
@@ -505,6 +505,27 @@ fn shape_builder_click_on_a_side_of_a_line_splits_the_shape() {
     assert_eq!(open_paths(&s, &out), vec![(2, Rect::new(50.0, -20.0, 50.0, 120.0))]);
 }
 
+/// A line that ends inside a shape has the same region on both sides there: a click or a drag
+/// across it, or deleting the region, leaves it whole.
+#[test]
+fn shape_builder_merge_keeps_a_line_that_ends_inside_the_shape() {
+    let mut s = session();
+    let a = rect(&mut s, 0.0, 0.0, 100.0, 100.0);
+    set_fill(&mut s, a, Color::rgb(1.0, 0.0, 0.0));
+    let l = line(&mut s, (50.0, -20.0), (50.0, 99.0));
+    s.execute("select.set", &json!({"ids": [a.0, l.0]})).unwrap();
+    let mut lines = vec![];
+    for params in [json!({"points": [[25, 50]]}), json!({"points": [[25, 50], [75, 50]]}), json!({"points": [[25, 50]], "erase": true})] {
+        let r = s.execute("shapeBuilder.merge", &params).unwrap();
+        assert_eq!(r["regions"], 1, "{params}");
+        let merged = r["merged"].as_u64().map(|m| (area(&s, NodeId(m)) - 10000.0).abs() < 1.0);
+        assert_eq!(merged, params["erase"].is_null().then_some(true), "{params}");
+        lines.push(open_paths(&s, &ids(&r)));
+        s.execute("edit.undo", &json!({})).unwrap();
+    }
+    assert_eq!(lines, vec![vec![(2, Rect::new(50.0, -20.0, 50.0, 99.0))]; 3]);
+}
+
 #[test]
 fn shape_builder_merge_across_a_line_removes_the_piece_between() {
     let mut s = session();
diff --git a/crates/tools/src/builder.rs b/crates/tools/src/builder.rs
index 2142769ec..1a13609ef 100644
--- a/crates/tools/src/builder.rs
+++ b/crates/tools/src/builder.rs
@@ -479,10 +479,11 @@ impl BuilderMap {
         [m + off, m - off].map(|q| self.region_at(q))
     }
 
-    /// Does line piece `i` run between two of `regions` (an edge that merging or deleting them
-    /// removes)?
+    /// Does line piece `i` run between two different regions of `regions` (an edge that merging or
+    /// deleting them removes)? A piece with the same region on both sides, such as the end of a
+    /// line inside a shape, bounds nothing and stays.
     pub fn line_between(&self, i: usize, regions: &[usize]) -> bool {
-        self.lines.get(i).is_some_and(|(l, _)| self.sides(l).iter().all(|r| r.is_some_and(|r| regions.contains(&r))))
+        self.lines.get(i).is_some_and(|(l, _)| matches!(self.sides(l), [Some(a), Some(b)] if a != b && regions.contains(&a) && regions.contains(&b)))
     }
 
     /// The edges `touched` deletes: those that bound none of its regions (deleting a region takes