Perspective grid: a grid dragged off its preset reports no preset name

perspective.grid.get reports define.name only while the grid still has
the preset's fields, as Define Grid's Preset menu shows [Custom]. Part of #5248.

crates/engine/src/cmd/perspgrid.rs

@@ -29,7 +29,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Perspective Grid",
             [],
             None,
-            "{} → {grid: the model (as perspective.grid.set takes it), define: the Define Grid fields (as perspective.grid.define takes them), station: {x, y, distance} (the viewer: centre of vision on the horizon, distance to the picture plane, pt), defined: false while the document uses the default grid}",
+            "{} → {grid: the model (as perspective.grid.set takes it), define: the Define Grid fields (as perspective.grid.define takes them; name is the preset only while the grid still has its fields, else \"\" for custom: dragging the horizon, a vanishing point or the station point makes it custom, moving a plane does not), station: {x, y, distance} (the viewer: centre of vision on the horizon, distance to the picture plane, pt), defined: false while the document uses the default grid}",
             has_doc,
             grid_get
         ),
@@ -222,7 +222,13 @@ pub(crate) fn grid_info(doc: &vectorcraft_doc::Document, g: &PerspectiveGrid) ->
 
 fn grid_get(s: &mut Session, _: &Value) -> Result<Value> {
     let doc = &s.doc()?.doc;
-    Ok(grid_info(doc, &grid_of(doc)))
+    let g = grid_of(doc);
+    let mut info = grid_info(doc, &g);
+    // The grid is its preset only while it has the preset's fields: a station point or vanishing
+    // point dragged since makes it a custom grid.
+    let def = g.definition();
+    info["define"]["name"] = json!(s.preset_named(&def, &[&def.name]));
+    Ok(info)
 }
 
 fn grid_define(s: &mut Session, p: &Value) -> Result<Value> {

crates/engine/src/tests/perspgrid.rs

@@ -342,6 +342,23 @@ fn lock_station_point_swings_the_other_vanishing_point() {
     assert!(n.viewing_angle() < 45.0);
 }
 
+#[test]
+fn a_grid_dragged_off_its_preset_reports_no_preset_name() {
+    let mut s = session();
+    run(&mut s, "perspective.grid.preset", json!({"name": "[1P-Normal View]"}));
+    let name = |s: &mut Session| run(s, "perspective.grid.get", json!({}))["define"]["name"].clone();
+    assert_eq!(name(&mut s), json!("[1P-Normal View]"));
+    // Moving a plane keeps the definition: still the preset.
+    run(&mut s, "perspective.plane.move", json!({"plane": "ground", "by": 50}));
+    assert_eq!(name(&mut s), json!("[1P-Normal View]"));
+    let g = grid(&s);
+    drag(&mut s, "perspectiveGrid", Point::new(g.vp_left, g.horizon), Point::new(g.vp_left - 40.0, g.horizon - 60.0));
+    assert!(!near(grid(&s).horizon, g.horizon), "the horizon moved");
+    assert_eq!(name(&mut s), json!(""), "a custom grid now");
+    s.execute("edit.undo", &json!({})).unwrap();
+    assert_eq!(name(&mut s), json!("[1P-Normal View]"));
+}
+
 #[test]
 fn snap_to_grid_lands_drawn_and_moved_art_on_gridlines() {
     let mut s = session();

docs/mcp.md

@@ -6053,7 +6053,9 @@ a stroke when it has `rect`. With both `rect` and `points`, the command also tou
 ## Perspective grid
 
 The grid is document data (`perspective.grid.set` edits the model directly). `perspective.grid.get` reads it:
-`grid` (the model), `define` (the Define Grid fields), `station` (the viewer: `x` the centre of vision on the
+`grid` (the model), `define` (the Define Grid fields; its `name` is the preset only while the grid still has that
+preset's fields, else `""` for a custom grid, so dragging the horizon, a vanishing point or the station point clears it
+and moving a plane does not), `station` (the viewer: `x` the centre of vision on the
 horizon `y`, `distance` from the picture plane, in points) and `defined` (false while the document uses the default
 grid). `perspective.grid.define` sets the grid from the Define Grid fields: `kind`, `units`, `scale`
 (`[artboard, real world]`), `gridline`, `angle` (the viewing angle, 0–90°), `distance` (the viewing distance),