Mesh: a color at a mesh envelope's point records no undo step

A mesh envelope's points carry no color, so object.mesh.addLine with
a color at a point the envelope already has changes nothing. It now
compares only the envelope's points and handles and records no undo
step. The command text and docs/mcp.md say so.

crates/engine/src/cmd/live/commands.rs

@@ -290,7 +290,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Add Mesh Line",
             [],
             None,
-            "{id, x, y, color? (#rrggbbaa and #rgba are refused), keepModel?: bool (as paint.setFill), tolerance?: pt (0)} add a row and a column through (x, y), inside or on the edge, to a gradient mesh (new point in `color`) or a mesh envelope; a point outside within `tolerance` of the edge adds them through the nearest point of the edge; at a point that is already a mesh point it adds no line, and records no undo step unless `color` changes it → {index}",
+            "{id, x, y, color? (#rrggbbaa and #rgba are refused), keepModel?: bool (as paint.setFill), tolerance?: pt (0)} add a row and a column through (x, y), inside or on the edge, to a gradient mesh (new point in `color`) or a mesh envelope; a point outside within `tolerance` of the edge adds them through the nearest point of the edge; at a point that is already a mesh point it adds no line, and records no undo step unless `color` changes a gradient mesh's point (a mesh envelope's points have no color) → {index}",
             has_doc,
             mesh_add_line
         ),

crates/engine/src/cmd/live/mesh.rs

@@ -298,13 +298,16 @@ pub(super) fn mesh_add_line(s: &mut Session, p: &Value) -> Result<Value> {
     }
     // The lines are added to a copy of the object. A point that is already a mesh point adds none:
     // when its color stays too, nothing changes and no undo step is recorded. The grid is compared,
-    // not the node: a mesh envelope stored without handles takes them in any edit.
+    // not the node: a mesh envelope stored without handles takes them in any edit, and keeps no
+    // colors, so only its points and handles count.
     let orig = s.doc()?.doc.node(id).ok_or(EngineError::NoNode(id))?.kind.clone();
+    let colored = matches!(orig, NodeKind::Mesh(_));
     let mut kind = orig.clone();
     let (i, same) = edit_kind(&mut kind, C, true, |m| {
         let before = m.clone();
         let i = add(m)?;
-        Ok((i, *m == before))
+        let shape = |g: &GradientMesh| (g.rows, g.cols, g.points.iter().map(|q| (q.p, q.handles)).collect::<Vec<_>>());
+        Ok((i, if colored { *m == before } else { shape(m) == shape(&before) }))
     })?;
     if same {
         return Ok(json!({ "index": i }));

crates/engine/src/tests/envelope.rs

@@ -124,7 +124,8 @@ fn reset_with_warp_and_mesh_switch_the_kind_and_keep_the_content() {
 }
 
 /// `object.mesh.addLine` at a point a mesh envelope already has (one stored without handles) adds
-/// no line and records no undo step; a point between its lines adds them as one step.
+/// no line and records no undo step, with or without a `color`; a point between its lines adds
+/// them as one step.
 #[test]
 fn mesh_envelope_add_line_at_a_mesh_point_records_no_undo_step() {
     let mut s = session();
@@ -137,6 +138,9 @@ fn mesh_envelope_add_line_at_a_mesh_point_records_no_undo_step() {
         s.execute("object.mesh.addLine", &json!({"id": e.0, "x": x, "y": y})).unwrap();
         assert_eq!((kind(&s, e), undo_len(&s)), (before.clone(), n), "({x}, {y}): nothing changed");
     }
+    // An envelope's points carry no color, so a color there changes nothing either.
+    s.execute("object.mesh.addLine", &json!({"id": e.0, "x": 200, "y": 150, "color": "#00ff00"})).unwrap();
+    assert_eq!((kind(&s, e), undo_len(&s)), (before.clone(), n), "a color changes nothing");
     s.execute("object.mesh.addLine", &json!({"id": e.0, "x": 150, "y": 125})).unwrap();
     assert!(matches!(kind(&s, e), EnvelopeKind::Mesh { rows: 3, cols: 3, .. }), "{:?}", kind(&s, e));
     assert_eq!(undo_len(&s), n + 1);

docs/mcp.md

@@ -9628,7 +9628,8 @@ right/left/down/up; none stored means the smooth mesh through the points, as old
 shows its mesh; the Mesh tool and Direct Selection drag its points and the clicked point's handles, and the Mesh tool
 adds a row and a column where it clicks inside. `object.mesh.addLine`, and `object.mesh.create` with `at`, take a point exactly on
 the mesh's outline (within a millionth of a point) on any side without `tolerance`; a point outside needs `tolerance`. `object.mesh.addLine` at a point that is already a mesh
-point adds no line and records no undo step unless its `color` changes the point. `press_key` Escape during a drag of a point or handle of a mesh
+point adds no line and records no undo step unless its `color` changes a gradient mesh's point (a mesh envelope's points
+have no color). `press_key` Escape during a drag of a point or handle of a mesh
 envelope or a gradient mesh cancels it: the point goes back and no undo step is added. The Control bar shows Edit Envelope / Edit Contents, the warp's
 style, orientation, bend and distortions (or the mesh's rows and columns), Reset (an unbent warp, a flat mesh) and
 Envelope Options.