INSERT with explode records one undo step (#475) (#585)

Squash of PR #585 by @MarekSpodBiedry.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/engine/src/cmd/blocks.rs

@@ -225,7 +225,9 @@ fn run_insert(s: &mut Session, p: &Value) -> Result<Value> {
     let vals = p.get("attribs").and_then(Value::as_object).cloned().unwrap_or_default();
     let h = insert_with(s, &name, at, scale, rot, point_param(p, "basePoint"), &vals)?;
     if bool_or(p, "explode", false) {
-        return s.execute("explode", &json!({ "handles": [h.hex()] }));
+        // The EXPLODE command body, not `execute`: INSERT records the one undo step.
+        let explode = find_command("explode").ok_or_else(|| EngineError::UnknownCommand("explode".into()))?;
+        return (explode.run)(s, &json!({ "handles": [h.hex()] }));
     }
     Ok(attedit::with_unknown_tags(s, h, &vals, json!({ "handle": h.hex() })))
 }

crates/engine/tests/insert_file.rs

@@ -150,3 +150,27 @@ fn a_drawing_file_becomes_a_block_with_its_insbase_and_tables() {
     assert!(s.prompt_text().contains("block name"), "{}", s.prompt_text());
     assert_eq!(inserts(&s).len(), n);
 }
+
+#[test]
+fn exploded_json_insert_is_one_undo_step() {
+    for params in [json!({ "name": "*B", "at": [5, 5] }), json!({ "name": "B", "at": [5, 5], "explode": true })] {
+        let mut s = Session::new();
+        s.execute("line", &json!({ "points": [[0, 0], [10, 0]] })).unwrap();
+        s.execute("block", &json!({ "name": "B", "base": [0, 0], "keep": "delete", "handles": ["100"] })).unwrap();
+        let undo = s.state().unwrap().undo.len();
+        s.execute("insert", &params).unwrap();
+        assert_eq!(lines(&s), vec![(5.0, 5.0, 15.0, 5.0)], "{params}");
+        assert_eq!(s.state().unwrap().undo.len(), undo + 1, "{params}");
+
+        s.execute("undo", &json!({})).unwrap();
+        assert!(lines(&s).is_empty(), "{params}");
+        assert!(inserts(&s).is_empty(), "{params}");
+        assert!(s.doc().unwrap().block("B").is_some(), "{params}");
+
+        // The second undo reverts the block creation, not a leftover EXPLODE step.
+        s.execute("undo", &json!({})).unwrap();
+        assert!(inserts(&s).is_empty(), "{params}");
+        assert!(s.doc().unwrap().block("B").is_none(), "{params}");
+        assert_eq!(lines(&s), vec![(0.0, 0.0, 10.0, 0.0)], "{params}");
+    }
+}