M8: joining the ends of a path broken open closes its fill loop through the new segment (#1951)

Cause: deleting a point with Delete (not Shift+Delete) opens the path and leaves its region's
loop with a gap, filled closed by a straight line across it; joining the two ends again (the pen
onto the other end, Connect, Join) added the segment to the network but not to the loop, which
still spanned the gap. Unrounded, the straight line and the segment coincide; once the points are
rounded, the fill and the selection outline run straight across while the stroke, walked from the
network itself, follows the segment and its arcs (#1506's picture).

Fix: `ops::connect` splices the new segment into the loop whose segments are the chain between
the two ends, so the loop closes through it; a chain no loop fills still becomes a region of its
own. Tests: the chamfer made by Shift+Delete, by Delete then Connect, and drawn with the pen, each
rounded, with the fill tracing the stroke (engine); every loop order and segment direction of a
chamfered rectangle (doc).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

crates/doc/src/tests.rs

@@ -681,6 +681,61 @@ fn two_segment_closed_loop_rounded_corners() {
     assert!(!path.elements().is_empty());
 }
 
+/// A rectangle whose top-right corner was cut off in the vector editor (a diagonal between a point
+/// on the top edge and one on the right edge), then rounded: the fill, which the fill and the
+/// selection outline draw, is cut by an arc at every point just as the stroke is, however the loop
+/// lists its segments and whichever way each one runs (#1951, #1506).
+#[test]
+fn a_chamfered_rectangles_rounded_fill_is_cut_at_every_point() {
+    use figcraft_geom::BezPath;
+    use figcraft_geom::kurbo::{ParamCurveArclen, ParamCurveNearest};
+    let corners = [Point::new(0.0, 0.0), Point::new(50.0, 0.0), Point::new(100.0, 50.0), Point::new(100.0, 100.0), Point::new(0.0, 100.0)];
+    let keeps_off = |path: &BezPath, p: Point| path.segments().map(|s| s.nearest(p, 1e-9).distance_sq.sqrt()).fold(f64::INFINITY, f64::min);
+    let length = |path: &BezPath| path.segments().map(|s| s.arclen(1e-9)).sum::<f64>();
+    for first in 0..corners.len() {
+        for backwards in [false, true] {
+            for flipped in 0..(1u32 << corners.len()) {
+                let mut net = VectorNetwork::from_points(&corners, true);
+                for v in &mut net.vertices {
+                    v.radius = 43.0;
+                }
+                for (i, s) in net.segments.iter_mut().enumerate() {
+                    if flipped & (1 << i) != 0 {
+                        std::mem::swap(&mut s.a, &mut s.b);
+                        std::mem::swap(&mut s.ta, &mut s.tb);
+                    }
+                }
+                let lp = &mut net.regions[0].loops[0];
+                lp.rotate_left(first);
+                if backwards {
+                    lp.reverse();
+                }
+                let why = format!("loop from segment {first}{}, segments flipped {flipped:05b}", if backwards { " backwards" } else { "" });
+                let stroke = net.stroke_path();
+                let fill = net.fill_path();
+                let region = net.rounded().unwrap().region_path(0).unwrap();
+                for (i, p) in corners.iter().enumerate() {
+                    assert!(keeps_off(&stroke, *p) > 2.0, "{why}: the stroke is cut at point {i} ({:.2} from it)", keeps_off(&stroke, *p));
+                    assert!(keeps_off(&fill, *p) > 2.0, "{why}: the fill runs into point {i} ({:.2} from it)", keeps_off(&fill, *p));
+                    assert!(keeps_off(&region, *p) > 2.0, "{why}: the region runs into point {i} ({:.2} from it)", keeps_off(&region, *p));
+                }
+                assert!(
+                    (length(&fill) - length(&stroke)).abs() < 1e-6,
+                    "{why}: fill {} and stroke {} trace the same outline",
+                    length(&fill),
+                    length(&stroke)
+                );
+                assert!(
+                    (length(&region) - length(&stroke)).abs() < 1e-6,
+                    "{why}: region {} and stroke {} trace the same outline",
+                    length(&region),
+                    length(&stroke)
+                );
+            }
+        }
+    }
+}
+
 #[test]
 fn stored_variable_ids_resolve_only_their_exact_collection() {
     let mut vars = crate::proto_expr_tests::vars();

crates/engine/src/canvas/vectoredit/ops.rs

@@ -658,8 +658,16 @@ pub fn connect(net: &mut VectorNetwork, a: usize, b: usize, ta: (f64, f64), tb:
     net.segments.push(Segment { a, b, ta: finite_t(ta), tb: finite_t(tb) });
     let si = net.segments.len() - 1;
     if ends && let Some(mut lp) = path_between(net, b, a, si) {
-        let unfilled = !net.regions.iter().any(|r| r.loops.iter().flatten().any(|s| lp.contains(s)));
-        if unfilled {
+        let filled = net.regions.iter().any(|r| r.loops.iter().flatten().any(|s| lp.contains(s)));
+        if filled {
+            // A loop broken open at these ends (a point of it deleted, say) closes through the new
+            // segment, so the fill follows it instead of a straight line across the gap (#1951).
+            for l in net.regions.iter_mut().flat_map(|r| r.loops.iter_mut()) {
+                if l.len() == lp.len() && l.iter().all(|s| lp.contains(s)) {
+                    *l = std::iter::once(si).chain(lp.iter().copied()).collect();
+                }
+            }
+        } else {
             lp.insert(0, si);
             net.regions.push(Region { loops: vec![lp], winding: WindingRule::NonZero, fills: None, ..Default::default() });
         }

crates/engine/src/canvas/vectoredit/tests.rs

@@ -597,3 +597,85 @@ fn the_pen_shows_its_ghost_anchor_on_the_segment_under_it() {
     assert_eq!(net(&s, id).vertices.len(), 5, "the press added the point");
     assert_eq!(ve(&s).anchor, None, "the click ended the pen's path");
 }
+
+/// A rectangle's corner cut off in the editor (a point added on each edge next to it, the corner
+/// deleted with Shift+Delete so the two join) and the shape rounded: the fill, which the fill and
+/// the selection outline draw, traces the stroke's outline, cut at both ends of the diagonal (#1951).
+#[test]
+fn a_rounded_chamfer_cut_in_the_editor_fills_along_its_arcs() {
+    for top_first in [true, false] {
+        let mut s = Session::new();
+        let id = rect(&mut s);
+        // Splitting keeps the split edge's index, so the other edge's stays: top 0, right 1.
+        let (first, second) = if top_first { (0, 1) } else { (1, 0) };
+        ex(&mut s, "vector.edit.addVertex", json!({"segment": first, "t": 0.5}));
+        ex(&mut s, "vector.edit.addVertex", json!({"segment": second, "t": 0.5}));
+        let corner = vertex_at(&net(&s, id), 100.0, 0.0);
+        ex(&mut s, "vector.edit.select", json!({"vertices": [corner]}));
+        ex(&mut s, "vector.deleteHeal", json!({}));
+        let n = net(&s, id);
+        assert_eq!((n.vertices.len(), n.segments.len(), n.regions.len()), (5, 5, 1), "top first {top_first}: a pentagon");
+        ex(&mut s, "vector.edit.cornerRadius", json!({"radius": 43, "vertices": [0, 1, 2, 3, 4]}));
+        fill_traces_stroke(&s, id, &format!("top first {top_first}"));
+        // A region with a fill of its own is drawn through the region path and the planar normaliser.
+        if let NodeKind::Vector { network } = &mut s.doc_mut().unwrap().get_mut(id).unwrap().kind {
+            network.regions[0].fills = Some(vec![figcraft_color::Paint::solid(figcraft_color::Color::BLACK)]);
+        }
+        fill_traces_stroke(&s, id, &format!("top first {top_first}, region fill"));
+    }
+}
+
+/// The chamfer made other ways: the corner deleted plainly, which opens the path between the two
+/// new points, and the two connected again; or the pentagon drawn with the pen (#1951).
+#[test]
+fn a_rounded_chamfer_made_other_ways_fills_along_its_arcs() {
+    let mut s = Session::new();
+    let id = rect(&mut s);
+    ex(&mut s, "vector.edit.addVertex", json!({"segment": 0, "t": 0.5}));
+    ex(&mut s, "vector.edit.addVertex", json!({"segment": 1, "t": 0.5}));
+    let corner = vertex_at(&net(&s, id), 100.0, 0.0);
+    ex(&mut s, "vector.edit.select", json!({"vertices": [corner]}));
+    ex(&mut s, "vector.edit.delete", json!({}));
+    let n = net(&s, id);
+    assert_eq!((n.segments.len(), n.regions[0].loops[0].len()), (4, 4), "the path opened: {:?}", n.regions);
+    let (top, right) = (vertex_at(&n, 50.0, 0.0), vertex_at(&n, 100.0, 50.0));
+    ex(&mut s, "vector.edit.connect", json!({"a": top, "b": right}));
+    assert_eq!(net(&s, id).segments.len(), 5, "connected again");
+    ex(&mut s, "vector.edit.cornerRadius", json!({"radius": 43, "vertices": [0, 1, 2, 3, 4]}));
+    fill_traces_stroke(&s, id, "deleted and connected");
+    // Drawn with the pen: continue from the end of an open two-point path and close on its start.
+    let mut s = Session::new();
+    let r = ex(&mut s, "vector.create", json!({"points": [[0, 0], [50, 0]], "closed": false}));
+    let id = crate::params::id_of(&r["id"]).unwrap();
+    ex(&mut s, "vector.edit.begin", json!({"id": id}));
+    ex(&mut s, "vector.edit.tool", json!({"tool": "pen"}));
+    click(&mut s, 50.0, 0.0);
+    for (x, y) in [(100.0, 50.0), (100.0, 100.0), (0.0, 100.0), (0.0, 0.0)] {
+        click(&mut s, x, y);
+    }
+    let n = net(&s, id);
+    assert_eq!((n.vertices.len(), n.segments.len(), n.regions.len()), (5, 5, 1), "closed by the pen");
+    ex(&mut s, "vector.edit.cornerRadius", json!({"radius": 43, "vertices": [0, 1, 2, 3, 4]}));
+    fill_traces_stroke(&s, id, "drawn with the pen");
+}
+
+/// The rounded chamfered rectangle `id` (five points on the origin's 100×100 box, the top-right
+/// corner cut off between (50, 0) and (100, 50)): one region with every segment in its loop, and a
+/// fill that keeps off every sharp point and traces the same outline as the stroke.
+fn fill_traces_stroke(s: &Session, id: NodeId, why: &str) {
+    use figcraft_geom::BezPath;
+    use figcraft_geom::kurbo::{ParamCurveArclen, ParamCurveNearest};
+    let n = net(s, id);
+    assert_eq!(n.regions.len(), 1, "{why}: one region, {:?}", n.regions);
+    assert_eq!(n.regions[0].loops[0].len(), n.segments.len(), "{why}: every segment in the loop, {:?}", n.regions[0].loops);
+    let doc = s.doc().unwrap();
+    let (fill, _) = figcraft_render::node_outline(doc, doc.node(id).unwrap()).unwrap();
+    let stroke = n.stroke_path();
+    let keeps_off = |path: &BezPath, p: Point| path.segments().map(|s| s.nearest(p, 1e-9).distance_sq.sqrt()).fold(f64::INFINITY, f64::min);
+    let length = |path: &BezPath| path.segments().map(|s| s.arclen(1e-9)).sum::<f64>();
+    for p in [pt(50.0, 0.0), pt(100.0, 50.0), pt(0.0, 0.0), pt(100.0, 100.0), pt(0.0, 100.0)] {
+        assert!(keeps_off(&stroke, p) > 2.0, "{why}: the stroke is cut at {p:?} ({:.2} from it)", keeps_off(&stroke, p));
+        assert!(keeps_off(&fill, p) > 2.0, "{why}: the fill runs into {p:?} ({:.2} from it)", keeps_off(&fill, p));
+    }
+    assert!((length(&fill) - length(&stroke)).abs() < 1e-3, "{why}: fill {} and stroke {} trace the same outline", length(&fill), length(&stroke));
+}