Selection: a key object needs at least two selected objects

crates/engine/src/cmd/edit.rs

@@ -278,6 +278,9 @@ pub fn specs() -> Vec<CommandSpec> {
             if !st.selection.items.contains(&id) {
                 return Err(super::bad("selection.key", "the key object must be selected"));
             }
+            if st.selection.items.len() < 2 {
+                return Err(super::bad("selection.key", "a key object needs at least two selected objects"));
+            }
             st.selection.key = if st.selection.key == Some(id) { None } else { Some(id) };
             st.bump_revision();
             ok()

crates/engine/src/tests.rs

@@ -1713,6 +1713,20 @@ fn deselecting_the_key_object_drops_the_key() {
     assert_eq!(s.doc().unwrap().selection.key, None);
 }
 
+/// A key object needs another object to align to: with one object selected there is nothing to key
+/// against, and the stored key would turn that object into the target as soon as a second one joins.
+#[test]
+fn a_single_selected_object_cannot_be_the_key_object() {
+    let mut s = session();
+    let a = s.execute("frame.create", &json!({"rect": [50.0, 100.0, 100.0, 150.0]})).unwrap()["id"].as_u64().unwrap();
+    let b = s.execute("frame.create", &json!({"rect": [200.0, 120.0, 300.0, 170.0]})).unwrap()["id"].as_u64().unwrap();
+    s.execute("selection.set", &json!({"ids": [a]})).unwrap();
+    assert!(s.execute("selection.key", &json!({"id": a})).is_err());
+    assert_eq!(s.doc().unwrap().selection.key, None);
+    s.execute("selection.toggle", &json!({"id": b})).unwrap();
+    assert_eq!(s.doc().unwrap().selection.key, None, "the second object joining must not find a key already set");
+}
+
 /// With the selection down to the key object alone, Align no longer has a key object: a single
 /// object aligns to the page, as before.
 #[test]