From 403997f0b36c5de0c1a8d8d2f99872948b10a816 Mon Sep 17 00:00:00 2001
From: Petru Conduraru <petru@bimvoice.com>
Date: Thu, 16 Jul 2026 13:26:16 +0300
Subject: [PATCH] bonsai: preserve original tessellation edges from the CGAL
 kernel

Fixes #5711. Also relevant to #5862 and #6747.

Root cause: tool.Loader.create_settings() builds the geometry settings
used for every import without enabling the CGAL kernel's
cgal-original-edges option. Bonsai's default geometry library is
hybrid-cgal-simple-opencascade, so most elements are triangulated by
CGAL, which groups all mutually adjacent coplanar triangles into a
single face boundary and only reports edges between non-coplanar
components. Any edge between two distinct but coplanar source faces
(for example a tessellation deliberately split into two coplanar
quads) is therefore never recorded in ios_edges, so
Geometry.dissolve_triangulated_edges silently merges those faces into
one n-gon the moment the object is edited, even though nothing in the
IFC file asked for that simplification.

Enabling cgal-original-edges makes the kernel report genuine face
boundaries instead of grouping by coplanarity. Verified live in
headless Blender 5.2 with a synthetic tessellated wall split into two
coplanar quads: before the fix the imported mesh collapsed to a
6-vertex ngon plus a quad (10 edges), after the fix it kept 3 separate
quads (11 edges), matching the OpenCASCADE kernel's output.

Also verified against a synthetic wall with an opening (the case
aothms flagged as risky, since booleans/voids have no meaningful
original topology to preserve) using Bonsai's actual default
hybrid-cgal-simple-opencascade library end to end: face and edge
counts were identical with and without the setting, because that
library already falls back to OpenCASCADE for boolean/void geometry,
so this change does not touch that path. Note the caveat does still
apply if a user explicitly switches Geometry Library to plain CGAL on
a model with lots of openings; a synthetic-only check of that specific
combination showed more exposed triangulation edges there.

Generated with the assistance of an AI coding tool.
---
 src/bonsai/bonsai/tool/loader.py | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/bonsai/bonsai/tool/loader.py b/src/bonsai/bonsai/tool/loader.py
index 383be821e2..f950f82ff7 100644
--- a/src/bonsai/bonsai/tool/loader.py
+++ b/src/bonsai/bonsai/tool/loader.py
@@ -727,6 +727,15 @@ class Loader(bonsai.core.tool.Loader):
             settings.set("layerset-first", True)
             # Wire intersection checks is prohibitively slow on advanced breps. See bug #5999.
             settings.set("no-wire-intersection-check", True)
+            # By default the CGAL kernel groups adjacent coplanar triangles into a
+            # single face boundary, discarding edges between distinct but coplanar
+            # source faces (e.g. a tessellation deliberately split into two
+            # coplanar quads). This makes those edges vanish when entering edit
+            # mode (see #5711, #5862). Preserve the original face boundaries
+            # instead. This is a no-op for the OpenCASCADE kernel and, in the
+            # default hybrid-cgal-simple-opencascade library, booleans/voids are
+            # resolved via the OpenCASCADE fallback so this does not affect them.
+            settings.set("cgal-original-edges", True)
             # settings.set("triangulation-type", ifcopenshell.ifcopenshell_wrapper.POLYHEDRON_WITHOUT_HOLES)
             if is_gross:
                 settings.set("disable-opening-subtractions", True)