Layers: Cmd/Ctrl+Alt-click places a new layer below the current one and a new sublayer at the bottom

The reference app places a new layer below the selected layer with
Ctrl+Alt-click (Command+Option-click) on the New Layer button, and a
new sublayer at the bottom of the layer list with the same click on
the New Sublayer button. In VectorCraft that click opened Layer
Options, and New Layer and New Sublayer always added the layer above
the current layer and the sublayer on top of the layer's contents.

layer.new takes below, which adds the layer directly below the current
layer at its level, and layer.newSublayer takes bottom, which adds the
sublayer under the parent's contents. In a clipping layer the new
sublayer goes above the clipping path. Each is one undo step, and
below is refused together with top. A Cmd/Ctrl+Alt-click on the
Layers panel's New Layer and New Sublayer buttons runs them. A plain
click, an Alt-click and a Cmd/Ctrl-click work as before.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/tests/layers.rs

@@ -403,6 +403,58 @@ fn new_layers_go_above_the_current_one_at_its_level() {
     assert_eq!(unique.len(), names.len(), "{names:?}");
 }
 
+/// `below` puts a new layer directly below the current layer, at its level, and `bottom` puts a
+/// new sublayer under the parent's contents, each in one undo step.
+#[test]
+fn new_layers_go_below_the_current_one_and_sublayers_under_the_contents() {
+    let mut s = session();
+    let (layer, sub, a, g, _, c) = tree(&mut s);
+    run(&mut s, "layer.setCurrent", json!({"id": sub.0}));
+    let before = undos(&s);
+    let n = id(&run(&mut s, "layer.new", json!({"below": true})));
+    assert_eq!(children(&s, layer), vec![c, n, sub], "a sublayer directly below the current sublayer");
+    assert_eq!(undos(&s), before + 1);
+    assert_eq!(s.doc().unwrap().highlighted_rows(), vec![n]);
+    assert_eq!(s.doc().unwrap().current_layer(), Some(n));
+    run(&mut s, "edit.undo", json!({}));
+    assert_eq!(children(&s, layer), vec![c, sub]);
+    // At the top level: below the current layer, above the layers under it.
+    let top = id(&run(&mut s, "layer.new", json!({"top": true, "name": "Top"})));
+    let middle = id(&run(&mut s, "layer.new", json!({"below": true, "name": "Middle", "locked": true})));
+    let order = s.doc().unwrap().doc.layers.iter().map(|l| l.id).collect::<Vec<_>>();
+    assert_eq!(order, vec![layer, middle, top]);
+    assert!(node(&s, middle).locked, "options as without `below`");
+    run(&mut s, "layer.setCurrent", json!({"id": layer.0}));
+    let bottom = id(&run(&mut s, "layer.new", json!({"below": true})));
+    assert_eq!(s.doc().unwrap().doc.layers[0].id, bottom, "below the bottom layer");
+    // Sublayers: on top of the parent's contents, or under them with `bottom`.
+    let before = undos(&s);
+    let under = id(&run(&mut s, "layer.newSublayer", json!({"parent": sub.0, "bottom": true})));
+    assert_eq!(children(&s, sub), vec![under, a, g]);
+    assert_eq!(undos(&s), before + 1);
+    assert_eq!(s.doc().unwrap().highlighted_rows(), vec![under]);
+    let over = id(&run(&mut s, "layer.newSublayer", json!({"parent": sub.0, "bottom": false})));
+    assert_eq!(children(&s, sub), vec![under, a, g, over]);
+    // Bad values change nothing.
+    let count = s.doc().unwrap().doc.node_count();
+    for (cmd, p) in [
+        ("layer.new", json!({"below": true, "top": true})),
+        ("layer.new", json!({"below": "yes"})),
+        ("layer.newSublayer", json!({"parent": sub.0, "bottom": 1})),
+    ] {
+        assert!(s.execute(cmd, &p).is_err(), "{cmd} {p}");
+    }
+    assert_eq!(s.doc().unwrap().doc.node_count(), count);
+    // In a clipping layer the clipping path stays at the bottom.
+    let mut s = session();
+    let layer = s.doc().unwrap().doc.layers[0].id;
+    let (back, front) = (rect(&mut s, 0.0), rect(&mut s, 100.0));
+    run(&mut s, "layer.clippingMask.toggle", json!({"id": layer.0}));
+    assert_eq!(children(&s, layer), vec![front, back], "the top object clips");
+    let under = id(&run(&mut s, "layer.newSublayer", json!({"parent": layer.0, "bottom": true})));
+    assert_eq!(children(&s, layer), vec![front, under, back]);
+}
+
 #[test]
 fn rows_move_above_below_and_into_layers_and_groups() {
     let mut s = session();