test: failing regression test for #262: sheet.fold: bend zone reaching the blank outline is reported as a slanted or curved edge (#277)

* test: reproduce #262 (sheet.fold: bend zone reaching the blank outline is reported as a slanted or curved edge)

* fix(sheet): name the bend zone reaching an outline corner instead of blaming a slanted edge (#262)

A fold whose bend zone contains a corner of the panel outline (a fold at a
tab's root) was reported as crossing a slanted or curved edge. Name the
corner and how far to move the fold line to clear it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RjGw4gA5ZvKrU8MQP6sBgT

---------

Co-authored-by: bherbruck <[email protected]>
Co-authored-by: Claude Opus 5.5 <[email protected]>

crates/doc/src/sheet.rs

@@ -1383,6 +1383,19 @@ impl SheetBody {
         let us: Vec<f64> = strip.iter().map(|q| q.dot(d)).collect();
         let (u0, u1) = (us.iter().cloned().fold(f64::MAX, f64::min), us.iter().cloned().fold(f64::MIN, f64::max));
         if ((u1 - u0) * ba - signed_area(&strip).abs()).abs() > 1e-6 * (u1 - u0).max(1.0) * ba.max(1.0) {
+            // A corner of the outline inside the zone (e.g. a fold right at a tab's root): name
+            // it and how far the line has to move to clear it.
+            let corners: Vec<(Vec2, f64)> = pts.iter().map(|q| (*q, q.dot(n) - s0)).filter(|(_, s)| *s > 1e-6 && *s < ba - 1e-6).collect();
+            if let Some(&(c, _)) = corners.first() {
+                let on = corners.iter().fold(0.0_f64, |m, (_, s)| m.max(*s));
+                let back = corners.iter().fold(0.0_f64, |m, (_, s)| m.max(ba - *s));
+                let w = apply_point(&self.world(panel), Vec3::new(c.x, c.y, self.t));
+                return Err(DocError::Invalid(format!(
+                    "the bend zone ({ba:.3} mm wide) reaches a corner of the sheet's outline at ({:.3}, {:.3}, {:.3}): \
+                     move the fold line at least {on:.3} mm toward the side that folds or {back:.3} mm toward the side that stays",
+                    w.x, w.y, w.z
+                )));
+            }
             return Err(DocError::Invalid("not supported yet: folds whose bend zone crosses a slanted or curved edge of the sheet".into()));
         }
         let up = !flip;

crates/engine/src/cmd/sheet_tests.rs

@@ -1558,3 +1558,29 @@ fn partial_depth_intersect_sub_extrusion_resolution_layers_refuse_atomically() {
         assert!((solvecraft_kernel::measure(&s.world_state().body(&name).unwrap().body).unwrap().volume - 15000.0).abs() < 1e-5);
     }
 }
+
+/// A 0.8 mm steel rule and a base flange from a closed outline on XY (the sheet spans z 0..0.8).
+fn outline_blank(s: &mut Session, outline: Value) {
+    run(s, "sheet.manage_rules", json!({"name": "Steel 0.8", "thickness": "0.8", "active": true}));
+    run(s, "sketch.create", json!({"plane": "XY", "name": "Blank"}));
+    run(s, "sketch.line", json!({"points": outline, "closed": true}));
+    run(s, "sketch.finish", json!({}));
+    run(s, "sheet.flange", json!({"sketch": "Blank"}));
+}
+
+/// A tab whose fold line sits exactly at its root: the bend zone reaches the blank outline beside
+/// the tab. Every edge is straight, so the error must not blame a slanted or curved edge (#262).
+#[test]
+fn fold_at_a_tab_root_names_the_real_cause() {
+    let mut s = Session::default();
+    outline_blank(&mut s, json!([[0, 0], [100, 0], [100, 50], [60, 50], [60, 70], [40, 70], [40, 50], [0, 50]]));
+    if let Err(e) = s.execute("sheet.fold", &json!({"points": [[40, 50, 0.8], [60, 50, 0.8]], "fixed": [50, 25, 0.8]})) {
+        let e = e.to_string();
+        assert!(!e.contains("slanted or curved"), "all edges are straight: {e}");
+        assert!(e.contains("bend zone"), "name the bend zone reaching the outline: {e}");
+    }
+    // 1 mm into the tab the same fold works.
+    let mut ok = Session::default();
+    outline_blank(&mut ok, json!([[0, 0], [100, 0], [100, 50], [60, 50], [60, 70], [40, 70], [40, 50], [0, 50]]));
+    run(&mut ok, "sheet.fold", json!({"points": [[40, 51, 0.8], [60, 51, 0.8]], "fixed": [50, 25, 0.8]}));
+}