Align arc creation and editing behavior

src/modules/draw/modify/break_cmd.rs

@@ -96,9 +96,15 @@ fn break_arc(arc: &ArcEnt, p1: DVec3, p2: DVec3) -> Vec<EntityType> {
     let cy = arc.center.y;
     let r = arc.radius;
 
-    // Project p1 and p2 onto the arc (world XY plane)
-    let a1 = angle_on_arc(cx, cy, p1);
-    let a2 = angle_on_arc(cx, cy, p2);
+    let plane = crate::entities::curve::arc_curve(arc).plane;
+    let Some(q1) = plane.project(p1.to_array()) else {
+        return vec![EntityType::Arc(arc.clone())];
+    };
+    let Some(q2) = plane.project(p2.to_array()) else {
+        return vec![EntityType::Arc(arc.clone())];
+    };
+    let a1 = angle_on_arc(cx, cy, DVec3::new(q1[0], q1[1], 0.0));
+    let a2 = angle_on_arc(cx, cy, DVec3::new(q2[0], q2[1], 0.0));
 
     let start = arc.start_angle;
     let end = arc.end_angle;

src/modules/draw/modify/lengthen.rs

@@ -213,33 +213,20 @@ fn lengthen_line(line: &LineEnt, pick_pt: Vec3, mode: &LenMode) -> Option<Entity
 }
 
 fn lengthen_arc(arc: &ArcEnt, pick_pt: Vec3, mode: &LenMode) -> Option<EntityType> {
-    let cx = arc.center.x as f32;
-    let cy = arc.center.y as f32;
-
     // Current arc span
     let span = arc_span_rad(arc.start_angle, arc.end_angle);
     let current_arc_len = arc.radius * span;
 
     let new_arc_len = apply_mode(current_arc_len, mode)?;
-    if new_arc_len < 1e-10 {
+    if new_arc_len < 1e-10 || new_arc_len >= std::f64::consts::TAU * arc.radius - 1.0e-9 {
         return None;
     }
     let new_span = new_arc_len / arc.radius;
 
     // Which end (start or end angle) is closer to pick?
-    let start_rad = arc.start_angle;
-    let end_rad = arc.end_angle;
-
-    let start_pt = Vec3::new(
-        cx + arc.radius as f32 * start_rad.cos() as f32,
-        pick_pt.y,
-        cy + arc.radius as f32 * start_rad.sin() as f32,
-    );
-    let end_pt = Vec3::new(
-        cx + arc.radius as f32 * end_rad.cos() as f32,
-        pick_pt.y,
-        cy + arc.radius as f32 * end_rad.sin() as f32,
-    );
+    let curve = crate::entities::curve::arc_curve(arc);
+    let start_pt = Vec3::from_array(curve.point_at(0.0).map(|value| value as f32));
+    let end_pt = Vec3::from_array(curve.point_at(1.0).map(|value| value as f32));
     let dist_start = (pick_pt - start_pt).length();
     let dist_end = (pick_pt - end_pt).length();
 

src/modules/draw/modify/offset.rs

@@ -127,8 +127,8 @@ fn offset_circle(c: &CircleEnt, dist: f64, side_pt: Vec3) -> Option<EntityType>
 // ── Arc offset ─────────────────────────────────────────────────────────────
 
 fn offset_arc(a: &ArcEnt, dist: f64, side_pt: Vec3) -> Option<EntityType> {
-    let px = side_pt.x as f64;
-    let py = side_pt.y as f64;
+    let plane = crate::entities::curve::arc_curve(a).plane;
+    let [px, py] = plane.project(side_pt.as_dvec3().to_array())?;
     let dc = ((px - a.center.x).powi(2) + (py - a.center.y).powi(2)).sqrt();
 
     let new_r = if dc < a.radius {