POLYGON: an Edge second endpoint on the first is refused and asked again (follow-up to #735) (#760)

crates/engine/src/cmd/draw.rs

@@ -895,6 +895,9 @@ impl Interactive for PolygonM {
         let n = self.sides.unwrap_or(4);
         if self.edge {
             if let Input::Point(p) = i {
+                if self.edge_pts.first().is_some_and(|a| a.near(p, 1e-12)) {
+                    return Err(crate::EngineError::Other("Edge endpoints must differ.".into()));
+                }
                 self.edge_pts.push(p);
                 if self.edge_pts.len() == 2 {
                     s.add_entity(lwpoly(polygon_from_edge(self.edge_pts[0], self.edge_pts[1], n), true))?;

crates/engine/tests/draw_prompts.rs

@@ -65,6 +65,20 @@ fn polygon_typed_radius_has_a_horizontal_bottom_edge() {
     }
 }
 
+#[test]
+fn polygon_edge_endpoints_must_differ() {
+    let mut s = Session::new();
+    s.cmdline("polygon 4 e 1,1 1,1").unwrap();
+    assert_eq!(s.doc().unwrap().model.len(), 0, "no polygon");
+    assert!(logged(&s, "Edge endpoints must differ."), "{:?}", s.log);
+    assert!(s.running.is_some(), "still at the second endpoint prompt");
+    s.cmdline("3,1").unwrap();
+    assert!(s.running.is_none(), "a distinct endpoint finishes");
+    let EntityKind::LwPolyline(p) = last(&s) else { panic!("no polygon") };
+    assert!(p.closed && p.vertices.len() == 4);
+    assert!(p.vertices.iter().any(|v| v.p.near(Vec2::new(3.0, 1.0), 1e-9)), "the edge runs from 1,1 to 3,1");
+}
+
 #[test]
 fn circle_center_prompt_rejects_garbage() {
     let mut s = Session::new();