Align: a rejected select.key keeps the key anchor that was set

select.key {id, anchor} set the new key anchor before checking it, so a
point that is in the selection but not on the path cleared the key
anchor and then returned an error. The point is now checked first: an
anchor the path does not have is not a selected anchor point, and the
other conditions are checked on a copy of the selection.

crates/engine/src/cmd/select.rs

@@ -670,14 +670,20 @@ fn key_anchor(s: &mut Session, id: Option<NodeId>, guide: Option<usize>, v: &Val
         return Err(bad(C, "give `anchor` or `guide`, not both"));
     }
     let id = id.ok_or_else(|| bad(C, "`anchor` needs `id`, the path the point is on"))?;
-    let sel = &s.doc()?.selection;
-    if sel.segments.contains_key(&id) || !sel.anchors.get(&id).is_some_and(|set| set.contains(&r)) {
+    let st = s.doc()?;
+    let sel = &st.selection;
+    let on_path = st.doc.node(id).and_then(|n| n.path_data()).is_some_and(|p| p.subpaths.get(r.0).is_some_and(|sp| r.1 < sp.anchors.len()));
+    if !on_path || sel.segments.contains_key(&id) || !sel.anchors.get(&id).is_some_and(|set| set.contains(&r)) {
         return Err(bad(C, format!("anchor [{}, {}] of object {id} is not a selected anchor point", r.0, r.1)));
     }
-    s.select(|_, sel| sel.key_anchor = Some((id, r)))?;
-    if s.doc()?.selection.key_anchor.is_none() {
+    // Check the new key on a copy first, so a request that fails leaves the key as it was.
+    let mut trial = sel.clone();
+    trial.key_anchor = Some((id, r));
+    trial.prune(&st.doc);
+    if trial.key_anchor.is_none() {
         return Err(bad(C, "select another anchor point to align to the key anchor"));
     }
+    s.select(|_, sel| sel.key_anchor = Some((id, r)))?;
     key_result(s)
 }
 

crates/engine/src/tests/keyanchor.rs

@@ -147,3 +147,16 @@ fn select_key_rejects_an_anchor_that_cannot_be_the_key() {
     assert!(err(&mut s, json!({"id": a.0, "anchor": [0, 0]})).contains("is not a selected anchor point"));
     assert_eq!(key_anchor(&s), None, "the key stays as it was");
 }
+
+#[test]
+fn a_rejected_select_key_keeps_the_key_anchor() {
+    let mut s = session();
+    let a = path(&mut s, "M100 40 L200 60 L150 160 Z");
+    s.execute("select.anchors", &json!({"id": a.0, "anchors": [[0, 0], [0, 9]], "mode": "set"})).unwrap();
+    s.execute("select.anchors", &json!({"id": a.0, "anchors": [[0, 1]], "mode": "add"})).unwrap();
+    assert_eq!(key_anchor(&s), Some((a, (0, 1))));
+    // [0, 9] is in the selection but not on the path.
+    let err = s.execute("select.key", &json!({"id": a.0, "anchor": [0, 9]})).expect_err("a point the path does not have").to_string();
+    assert!(err.contains("is not a selected anchor point"), "{err}");
+    assert_eq!(key_anchor(&s), Some((a, (0, 1))), "the key stays as it was");
+}