Sketch: always show a grid on the open sketch's plane

While a sketch is open its grid is drawn on the sketch's own plane, centred
near the view target and snapped to the major step, whatever the Grid switch,
the sketch's Sketch Grid option or Grid follows the view say. Headless window
pictures follow. Outside a sketch nothing changes.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

crates/engine/src/view.rs

@@ -349,6 +349,20 @@ pub fn scene_with_section(s: &Session, cam: &Camera, section: Option<(Vec3, Vec3
     scene_with_guides(s, cam, section, Guides::default())
 }
 
+/// The plane of the sketch that is open, if any: it always shows a grid, whatever the Grid switch
+/// says (outside a sketch the grid is the user's choice).
+pub fn open_sketch_plane(s: &Session) -> Option<Plane> {
+    let id = s.active_sketch?;
+    s.world_state().sketch(id).map(|ss| ss.plane)
+}
+
+/// The grid on a sketch plane: its centre (the view target's foot on the plane, snapped to the
+/// major step in the plane's own axes) and the plane's x and y directions.
+pub fn sketch_grid_frame(plane: &Plane, target: Vec3, major: f64) -> (Vec3, Vec3, Vec3) {
+    let snap = |d: Vec3| d * (((target - plane.origin).dot(d) / major).round() * major);
+    (plane.origin + snap(plane.x) + snap(plane.y), plane.x, plane.y)
+}
+
 /// What a headless render draws besides the model: the ground grid and the origin's axes.
 /// Both are on by default; the grid lies on Z 0, so it cuts through a model that straddles it.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -434,12 +448,17 @@ fn build_scene(
     if guides.grid {
         let (minor, major) = grid_step(cam.half_height());
         let ext = (cam.half_height() * 3.0 / major).ceil() * major;
-        let tf = cam.to_frame(cam.target);
-        // Headless pictures keep the grid on the ground, the viewport's default.
-        let (u, v) = (Vec3::X, Vec3::Y);
-        let snap = |a: Vec3| a * ((tf.dot(a) / major).round() * major);
-        let c = snap(u) + snap(v);
-        let w = |p: Vec3| cam.from_frame(p);
+        // Headless pictures keep the grid on the ground, the viewport's default; an open sketch
+        // has its own plane's grid (world coordinates, no camera frame).
+        let (c, u, v, w): (Vec3, Vec3, Vec3, Box<dyn Fn(Vec3) -> Vec3>) = if let Some(pl) = open_sketch_plane(s) {
+            let (c, u, v) = sketch_grid_frame(&pl, cam.target, major);
+            (c, u, v, Box::new(|p| p))
+        } else {
+            let tf = cam.to_frame(cam.target);
+            let (u, v) = (Vec3::X, Vec3::Y);
+            let snap = |a: Vec3| a * ((tf.dot(a) / major).round() * major);
+            (snap(u) + snap(v), u, v, Box::new(|p| cam.from_frame(p)))
+        };
         let n = ((ext / minor) as i64).clamp(1, 400);
         for i in -n..=n {
             let t = i as f64 * minor;