#657: a drop that would leave no segment only moves the point

Dropping one end of a 2-point line on the other left a 1-point vector.

crates/engine/src/vectoredit/pointer.rs

@@ -1082,10 +1082,16 @@ impl Session {
         else {
             return Ok(());
         };
+        let mut merged = net.clone();
+        ops::merge_vertices(&mut merged, keep, v);
+        // A merge that would leave no segment (a 2-point line's ends dropped together) only moves.
+        if merged.segments.is_empty() {
+            return Ok(());
+        }
         // The preview starts again from the document before the drag: bring the moved layer.
         self.preview(|doc, _| {
             *doc.get_mut(id)? = moved;
-            edit(doc, id, |net| ops::merge_vertices(net, keep, v))
+            edit(doc, id, |net| *net = merged)
         })?;
         set_state(self, |e| {
             e.vertices = vec![if keep > v { keep - 1 } else { keep }];

crates/engine/src/vectoredit/tests.rs

@@ -300,6 +300,14 @@ fn a_point_dropped_on_another_merges_into_it() {
     ex(&mut s, "vector.edit.select", json!({"vertices": []}));
     drag(&mut s, (0.0, 100.0), (0.0, 50.0));
     assert_eq!(net(&s, id).vertices.len(), 4);
+    // A 2-point line's end dropped on its other end only moves: no segment would be left.
+    let mut s = Session::new();
+    let r = ex(&mut s, "vector.create", json!({"points": [[0, 0], [100, 0]]}));
+    let id = crate::params::id_of(&r["id"]).unwrap();
+    ex(&mut s, "vector.edit.begin", json!({"id": id}));
+    drag(&mut s, (100.0, 0.0), (2.0, 0.0));
+    let n = net(&s, id);
+    assert_eq!((n.vertices.len(), n.segments.len()), (2, 1), "still a line: {n:?}");
 }
 
 #[test]