A stray click must not break a model that looks perfect on screen

Revolve failed on a sketch whose profile was closed. Decoding the reported 3mf: four
entities forming a proper closed loop (joints open by 4.44e-06 mm, well inside
tolerance) plus one stray 1.82 mm Line at (-24.2, 80.3), inside the shaded region,
touching nothing.

The viewport's region_loops discards open chains ON PURPOSE — it exists to find
EXTRUDABLE regions — so the user saw one clean closed region. entities_to_wires kept
the stray as its own one-edge loop, so it returned two wires, and Revolve goes through
entities_to_wire which demands exactly one. Extrude would have failed one step later in
wires_to_face, because a one-edge open wire bounds no face. Same class as the tolerance
split fixed in 8b568b7b: the viewport and the kernel disagreeing about the sketch — this
time about what BELONGS to the profile.

entities_to_wires/entities_to_wire/build_sketch_wire take closed_only. It is not a
blanket rule: a SurfaceExtrude builds a sheet FROM an open profile and a Sweep PATH is
normally open, so all ten call sites are classified individually — true for the face
fallback, Extrude-taper, Revolve, the Sweep PROFILE and Loft profiles; false for
SurfaceExtrude/Revolve/Loft/Fill and the Sweep path.

A component counts as open when some welded node has DEGREE 1. The first attempt used
"the traversal did not return to its starting node", which regressed the bridged C
profile: a closed loop that also carries a second edge across the same two nodes has no
free endpoint, but its Eulerian walk ends elsewhere. Degree-1 is the property that
actually distinguishes a stray segment from a closed profile; the suite caught the
difference.

Behaviour change decided by Tommaso: a stray is IGNORED, not refused. The test that
required refusal dates from when ignoring meant falling through to a default rectangle —
geometry nobody drew. That fallback is gone, so ignoring now builds the circle the user
actually drew. Its assertion is updated with the reason.

The bridge round-trip test extruded an ENTIRELY open chain and "worked" only because
OCCT will make a face from an open wire. It gets a genuinely closed profile: the test is
about serialization, and deserialize_recipe recomputes, so the document has to be one
that legitimately builds.

Failures now say WHERE. sketch_open_ends reports free endpoints under the same weld
tolerance the wire build uses, and open_loop_message is shared by both throws, because
Extrude fails through build_sketch_face and Revolve through build_sketch_wire — enriching
only one would have left the commoner path the less informative one.

Kernel 66115 assertions / 608 cases green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FbJKJAJxxkhDTs9XdZzKA

tests/libslic3r/test_sketchedit.cpp

@@ -1,6 +1,7 @@
 #include <catch2/catch_all.hpp>   // mainline OrcaSlicer ships Catch2 v3 (v2 was catch2/catch.hpp)
 #include "libslic3r/CAD/SketchEngine.hpp"
 #include <cmath>
+#include <algorithm>
 #include <TopExp_Explorer.hxx>
 #include <TopAbs.hxx>
 
@@ -652,3 +653,78 @@ TEST_CASE("auto-close off makes the kernel demand an exact joint", "[SketchEngin
     // Restore the default so test order cannot leak OFF into the other cases.
     Slic3r::set_sketch_auto_close(true);
 }
+
+// A stray open segment touching nothing must not break a closed profile: the viewport
+// discards open chains when it shades a region extrudable, so with closed_only the kernel
+// must discard them too — otherwise Revolve/Extrude fail on a sketch that looks perfect.
+TEST_CASE("a stray open segment does not break a closed profile", "[SketchEngine]")
+{
+    auto line = [](double x0, double y0, double x1, double y1) {
+        SketchEntity e;
+        e.type = SketchEntity::Type::Line;
+        e.p0   = Vec2d(x0, y0);
+        e.p1   = Vec2d(x1, y1);
+        return e;
+    };
+
+    std::vector<SketchEntity> ents;
+    ents.push_back(line(0, 0, 20, 0));     // 20x10 quad
+    ents.push_back(line(20, 0, 20, 10));
+    ents.push_back(line(20, 10, 0, 10));
+    ents.push_back(line(0, 10, 0, 0));
+    ents.push_back(line(5, 5, 6, 5.2));    // stray, touches nothing
+
+    auto edge_count = [](const TopoDS_Wire& w) {
+        int n = 0;
+        for (TopExp_Explorer ex(w, TopAbs_EDGE); ex.More(); ex.Next()) ++n;
+        return n;
+    };
+
+    // Unchanged behaviour: the stray line is its own open wire.
+    auto wires_all = SketchEngine::entities_to_wires(ents, SketchPlane::XY(), /*closed_only=*/false);
+    REQUIRE(wires_all.size() == 2);
+
+    // closed_only drops the open chain: one closed quad survives.
+    auto wires_closed = SketchEngine::entities_to_wires(ents, SketchPlane::XY(), /*closed_only=*/true);
+    REQUIRE(wires_closed.size() == 1);
+    REQUIRE(edge_count(wires_closed[0]) == 4);
+    REQUIRE(wires_closed[0].Closed());
+
+    // The Revolve path (entities_to_wire) finds the single closed loop.
+    TopoDS_Wire w = SketchEngine::entities_to_wire(ents, SketchPlane::XY(), /*closed_only=*/true);
+    REQUIRE_FALSE(w.IsNull());
+    REQUIRE(edge_count(w) == 4);
+}
+
+// sketch_open_ends names the two free endpoints of an open chain, so the "does not form a
+// single closed wire" failure can say WHERE the sketch is open.
+TEST_CASE("sketch_open_ends names where a chain fails to close", "[SketchEngine]")
+{
+    auto line = [](double x0, double y0, double x1, double y1) {
+        SketchEntity e;
+        e.type = SketchEntity::Type::Line;
+        e.p0   = Vec2d(x0, y0);
+        e.p1   = Vec2d(x1, y1);
+        return e;
+    };
+
+    // Open C shape: three lines, free endpoints at (0,0) and (0,10).
+    std::vector<SketchEntity> ents;
+    ents.push_back(line(0, 0, 10, 0));
+    ents.push_back(line(10, 0, 10, 10));
+    ents.push_back(line(10, 10, 0, 10));
+
+    auto got = sketch_open_ends(ents, SketchPlane::XY());
+    REQUIRE(got.size() == 2);
+
+    std::sort(got.begin(), got.end(), [](const Vec2d& a, const Vec2d& b) {
+        if (a.x() < b.x()) return true;
+        if (a.x() > b.x()) return false;
+        return a.y() < b.y();
+    });
+
+    REQUIRE_THAT(got[0].x(), WithinAbs(0.0, 1e-9));
+    REQUIRE_THAT(got[0].y(), WithinAbs(0.0, 1e-9));
+    REQUIRE_THAT(got[1].x(), WithinAbs(0.0, 1e-9));
+    REQUIRE_THAT(got[1].y(), WithinAbs(10.0, 1e-9));
+}