Fix #340: new layers and sublayers share one "Layer N" numbering

A new sublayer was named "Layer {node_count}", counting every object, so
an empty document's first sublayer was "Layer 1" like its parent and a
document with five objects got "Layer 6". Top-level layers counted only
top-level names and could repeat a sublayer's. Document::next_layer_name
now counts every layer, sublayers included, and skips any layer's name;
layer.newSublayer uses it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/lib.rs

@@ -810,11 +810,19 @@ impl Document {
         self.layers.push(Arc::new(Node::layer(id, &name, LayerColor::Preset((n % LAYER_COLORS.len()) as u8))));
         id
     }
+    /// The default name of a new layer or sublayer, "Layer N". Layers and sublayers share the
+    /// numbering: it starts after the number of layers there are and skips any layer's name.
     pub fn next_layer_name(&self) -> String {
-        let mut i = self.layers.len() + 1;
+        let mut names = Vec::new();
+        self.walk(|n| {
+            if n.is_layer() {
+                names.push(n.name.as_deref().unwrap_or_default());
+            }
+        });
+        let mut i = names.len() + 1;
         loop {
             let name = format!("Layer {i}");
-            if !self.layers.iter().any(|l| l.name.as_deref() == Some(&name)) {
+            if !names.contains(&name.as_str()) {
                 return name;
             }
             i += 1;

crates/engine/src/tests.rs

@@ -570,3 +570,20 @@ fn a_panicking_command_rolls_back_instead_of_crashing() {
     assert!(s.doc().unwrap().doc.node(b).is_some());
     assert_eq!(s.journal.last().map(|j| j.0.as_str()), Some("shape.rectangle"));
 }
+
+/// New layers and sublayers share one "Layer N" numbering, whatever art the document holds, and
+/// never repeat a name.
+#[test]
+fn new_layers_and_sublayers_are_numbered_together() {
+    let name = |s: &Session, id: &Value| s.doc().unwrap().doc.node(NodeId(id["id"].as_u64().unwrap())).unwrap().display_name().to_string();
+    let mut s = session();
+    let sub = s.execute("layer.newSublayer", &json!({})).unwrap();
+    assert_eq!(name(&s, &sub), "Layer 2", "not its parent's name");
+    for x in [0.0, 60.0, 120.0, 180.0, 240.0] {
+        rect(&mut s, x, 0.0, 50.0, 50.0);
+    }
+    let layer = s.execute("layer.new", &json!({})).unwrap();
+    assert_eq!(name(&s, &layer), "Layer 3", "after the sublayer, not a repeat of it");
+    let sub = s.execute("layer.newSublayer", &json!({})).unwrap();
+    assert_eq!(name(&s, &sub), "Layer 4", "the art doesn't count");
+}