The viewport and the kernel now answer "is this joint closed?" with one number

Tommaso asked the question that names the real defect: if the sketch was open, why was
the same sketch shaded closed and offered for extrude? Because the two halves used
different tolerances. region_loops shades a region closed at 1e-3 mm; connected_loop
chained at 1e-3; the kernel welded at 1e-4 and OCCT matched vertices at 1e-7. The
2.28e-5 mm gap in the reported sketch did not cause that disagreement, it only made it
visible — and fixing the gap alone would have left the contradiction in place, ready to
reappear anywhere in (1e-4, 1e-3].

kSketchJoinTol now lives in SketchEngine.hpp and is the only place the number exists.
region_loops, loop_report, connected_loop and entities_to_wires all read it through
sketch_join_tol(). The viewport cannot promise a region the kernel refuses to build.

The welding is optional, because a kernel that silently closes loops should let you say
no: "Auto-close sketch loops" in Preferences, default ON, no restart. OFF means only
exactly coincident endpoints join — and since both halves read the same value, the
viewport simply stops shading the region closed, so an open loop is visible rather than
welded behind your back. No separate UI needed for that; it falls out of sharing one
number.

Details that matter. The kernel defaults to auto-close ON independently of the GUI, so
headless and MCP callers behave like the viewport instead of inheriting an unset
preference. With the tolerance at 0 the comparisons become <=, because OFF must mean
exact, not broken. OCCT never receives a zero vertex tolerance — it is clamped to
Precision::Confusion.

The preference is pushed from EVERY entry that starts a sketch session, not just
begin(): a Constrain session enters through begin_constrain / begin_constrain_entities
and uses region_loops and connected_loop, so a single push site would have left those
sessions running on whatever the previous one set. begin_imported_transform is excluded
deliberately — it works on imported regions, not chained entities.

Tests: a loop with one joint open by 9e-4 mm, given out of traversal order, builds a
closed four-edge wire; with auto-close off the same loop yields no wire; and an exactly
closed loop still builds with auto-close off, proving OFF means exact. Kernel 66104
assertions / 606 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

@@ -555,3 +555,100 @@ TEST_CASE("entities_to_wires keeps every edge of a loop drawn out of order", "[S
 
     REQUIRE(wires[0].Closed());
 }
+
+// Regression guard: this fails at 1e-4 (the wire builder refuses a joint the viewport had
+// already shaded closed) and passes at kSketchJoinTol. A 20x10 quad with one joint left open
+// by 9e-4 mm — just inside kSketchJoinTol, exactly the case the viewport shades closed — given
+// in an order that is NOT traversal order, so the ordering path is covered too.
+TEST_CASE("a loop the viewport shades closed is buildable by the kernel", "[SketchEngine]")
+{
+    std::vector<SketchEntity> ents(4);
+
+    // (0,0) -> (20,0) -> (20,10) -> (0,10) -> (0.0009, 0): last endpoint misses (0,0) by 9e-4.
+    ents[0].type = SketchEntity::Type::Line;
+    ents[0].p0   = Vec2d(0, 0);
+    ents[0].p1   = Vec2d(20, 0);
+
+    // Index 1 is the FAR side, not the neighbour of index 0: creation order here is
+    // deliberately not traversal order, so a partial wire would reject it without the
+    // traversal walk.
+    ents[1].type = SketchEntity::Type::Line;
+    ents[1].p0   = Vec2d(20, 10);
+    ents[1].p1   = Vec2d(0, 10);
+
+    ents[2].type = SketchEntity::Type::Line;
+    ents[2].p0   = Vec2d(20, 0);
+    ents[2].p1   = Vec2d(20, 10);
+
+    ents[3].type = SketchEntity::Type::Line;
+    ents[3].p0   = Vec2d(0, 10);
+    ents[3].p1   = Vec2d(0.0009, 0);
+
+    auto wires = SketchEngine::entities_to_wires(ents, SketchPlane::XY());
+
+    REQUIRE(wires.size() == 1);
+
+    int edge_count = 0;
+    for (TopExp_Explorer ex(wires[0], TopAbs_EDGE); ex.More(); ex.Next())
+        ++edge_count;
+    REQUIRE(edge_count == 4);
+
+    REQUIRE(wires[0].Closed());
+}
+
+// Regression guard for the auto-close preference. Same 20x10 quad, one joint open by 9e-4 mm
+// and given out of traversal order, as "a loop the viewport shades closed is buildable by the
+// kernel". With auto-close ON the gap welds (one closed wire); with auto-close OFF it must not.
+TEST_CASE("auto-close off makes the kernel demand an exact joint", "[SketchEngine]")
+{
+    std::vector<SketchEntity> ents(4);
+
+    ents[0].type = SketchEntity::Type::Line;
+    ents[0].p0   = Vec2d(0, 0);
+    ents[0].p1   = Vec2d(20, 0);
+
+    ents[1].type = SketchEntity::Type::Line;
+    ents[1].p0   = Vec2d(20, 10);
+    ents[1].p1   = Vec2d(0, 10);
+
+    ents[2].type = SketchEntity::Type::Line;
+    ents[2].p0   = Vec2d(20, 0);
+    ents[2].p1   = Vec2d(20, 10);
+
+    ents[3].type = SketchEntity::Type::Line;
+    ents[3].p0   = Vec2d(0, 10);
+    ents[3].p1   = Vec2d(0.0009, 0);
+
+    auto edge_count = [](const TopoDS_Wire& w) {
+        int n = 0;
+        for (TopExp_Explorer ex(w, TopAbs_EDGE); ex.More(); ex.Next()) ++n;
+        return n;
+    };
+
+    // ON: the 9e-4 mm gap is inside kSketchJoinTol, so the loop welds into one closed wire.
+    Slic3r::set_sketch_auto_close(true);
+    auto wires_on = SketchEngine::entities_to_wires(ents, SketchPlane::XY());
+    REQUIRE(wires_on.size() == 1);
+    REQUIRE(edge_count(wires_on[0]) == 4);
+    REQUIRE(wires_on[0].Closed());
+
+    // OFF: the joint is not exact, so the gap is NOT welded. entities_to_wires legitimately
+    // returns open chains (a sweep path is open), so the observable is an OPEN wire — the
+    // kernel no longer hands back the closed loop the viewport would have shaded.
+    Slic3r::set_sketch_auto_close(false);
+    auto wires_off = SketchEngine::entities_to_wires(ents, SketchPlane::XY());
+    REQUIRE(wires_off.size() == 1);
+    REQUIRE(edge_count(wires_off[0]) == 4);
+    REQUIRE_FALSE(wires_off[0].Closed());
+
+    // OFF + an EXACT joint (last endpoint exactly (0,0)): the quad still builds closed,
+    // proving "off" means exact rather than broken.
+    ents[3].p1 = Vec2d(0, 0);
+    auto wires_exact = SketchEngine::entities_to_wires(ents, SketchPlane::XY());
+    REQUIRE(wires_exact.size() == 1);
+    REQUIRE(edge_count(wires_exact[0]) == 4);
+    REQUIRE(wires_exact[0].Closed());
+
+    // Restore the default so test order cannot leak OFF into the other cases.
+    Slic3r::set_sketch_auto_close(true);
+}