Fix decorator face-tri overlay artifacts

ProfileDecorator.draw_faces (used by the roof path-edit overlay) and
SystemDecorator.draw_faces called bmesh.ops.triangulate on the live
bmesh — both mutated the input and produced ear-clip fans that rendered
as visible streaks across n-gon roof faces at alpha 0.1. The opening
DecorationsHandler edit-mode branch had a separate bug: it computed
triangles from obj.data.calc_loop_triangles() while iterating the
edit-mode bmesh, so any topology added mid-edit desynced the indices.

Centralise the correct draw path on tool.Blender.draw_bmesh_face_tris
(wraps bm.calc_loop_triangles, non-mutating, beauty triangulator) and
route all three call-sites through it. A forward-compat AST guard walks
every *Decorator / DecorationsHandler class under bim/module/ and pins
the no-bmesh.ops.triangulate rule against future regressions.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/model/decorator.py

@@ -15,6 +15,8 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was modified with the assistance of an AI coding tool.
 
 from __future__ import annotations
 
@@ -93,15 +95,9 @@ class ProfileDecorator:
         batch.draw(shader)
 
     def draw_faces(self, bm, vertices_coords):
-        """mutates original bm (triangulates it)
-        so the triangulation edges will be shown too
-        """
-        traingulated_bm = bm
-        bmesh.ops.triangulate(traingulated_bm, faces=traingulated_bm.faces)
-
-        face_indices = [[v.index for v in f.verts] for f in traingulated_bm.faces]
+        """Submit a non-mutating beauty-triangulated TRIS batch over ``bm``'s faces."""
         faces_color = transparent_color(self.addon_prefs.decorator_color_special)
-        self.draw_batch("TRIS", vertices_coords, faces_color, face_indices)
+        tool.Blender.draw_bmesh_face_tris(bm, vertices_coords, faces_color, self.draw_batch)
 
     def __call__(self, context, get_custom_bmesh=None, draw_faces=False, exit_edit_mode_callback=None):
         self.addon_prefs = tool.Blender.get_addon_preferences()

src/bonsai/bonsai/bim/module/model/opening.py

@@ -15,6 +15,8 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was modified with the assistance of an AI coding tool.
 
 from collections.abc import Sequence
 from math import radians
@@ -1278,9 +1280,7 @@ class DecorationsHandler:
                 self.draw_batch("LINES", verts, selected_elements_color, selected_edges)
                 self.draw_batch("POINTS", unselected_vertices, unselected_elements_color)
                 self.draw_batch("POINTS", selected_vertices, selected_elements_color)
-                obj.data.calc_loop_triangles()
-                tris = [tuple(t.vertices) for t in obj.data.loop_triangles]
-                self.draw_batch("TRIS", verts, transparent_color(special_elements_color), tris)
+                tool.Blender.draw_bmesh_face_tris(bm, verts, transparent_color(special_elements_color), self.draw_batch)
             else:
                 line_verts, verts, edges_indices, tris = _get_cached_world_draw_data(obj)
                 color = selected_elements_color if obj in context.selected_objects else special_elements_color

src/bonsai/bonsai/bim/module/system/decorator.py

@@ -15,9 +15,10 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was modified with the assistance of an AI coding tool.
 
 
-import bmesh
 import bpy
 import gpu
 from bpy.app.handlers import persistent
@@ -78,15 +79,9 @@ class SystemDecorator:
         batch.draw(shader)
 
     def draw_faces(self, bm, vertices_coords):
-        """mutates original bm (triangulates it)
-        so the triangulation edges will be shown too
-        """
-        traingulated_bm = bm
-        bmesh.ops.triangulate(traingulated_bm, faces=traingulated_bm.faces)
-
-        face_indices = [[v.index for v in f.verts] for f in traingulated_bm.faces]
+        """Submit a non-mutating beauty-triangulated TRIS batch over ``bm``'s faces."""
         faces_color = transparent_color(self.addon_prefs.decorator_color_special)
-        self.draw_batch("TRIS", vertices_coords, faces_color, face_indices)
+        tool.Blender.draw_bmesh_face_tris(bm, vertices_coords, faces_color, self.draw_batch)
 
     def __call__(self, context, get_custom_bmesh=None, draw_faces=False, exit_edit_mode_callback=None):
         self.addon_prefs = tool.Blender.get_addon_preferences()

src/bonsai/bonsai/tool/blender.py

@@ -2242,6 +2242,24 @@ class Blender(bonsai.core.tool.Blender):
             return False
         return True
 
+    @classmethod
+    def draw_bmesh_face_tris(
+        cls,
+        bm: bmesh.types.BMesh,
+        world_vert_coords: list,
+        color: Any,
+        draw_batch: Callable[[str, list, Any, list], None],
+    ) -> None:
+        """Submit a non-mutating beauty-triangulated TRIS batch for ``bm``'s faces.
+
+        ``world_vert_coords`` must be indexed by ``bm.verts`` index. Never call
+        ``bmesh.ops.triangulate`` on a live bmesh to compute draw indices — it
+        mutates the input and produces ear-clip fans that render as visible
+        streaks at low alpha.
+        """
+        tris = [[loop.vert.index for loop in tri] for tri in bm.calc_loop_triangles()]
+        draw_batch("TRIS", world_vert_coords, color, tris)
+
     @classmethod
     def extract_error_reports(cls, exception: RuntimeError) -> list[str]:
         """Extracts error report lines from a runtime exception during operator execution.

src/bonsai/test/bim/test_decorator_no_mutating_triangulate.py

@@ -0,0 +1,85 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+"""Forward-compat AST contract: decorators do not triangulate in-place.
+
+``bmesh.ops.triangulate(bm, faces=bm.faces)`` mutates its input — adding tri
+edges and faces — and uses ear-clip fan triangulation that renders as visible
+streaks across n-gon faces at the low alphas decorators favour. The canonical
+draw path is ``tool.Blender.draw_bmesh_face_tris`` (wraps ``bm.calc_loop_triangles``,
+non-mutating, beauty triangulator)."""
+
+import ast
+from pathlib import Path
+
+import pytest
+
+pytestmark = pytest.mark.model
+
+
+BONSAI_ROOT = Path(__file__).parent.parent.parent / "bonsai"
+BIM_MODULE_DIR = BONSAI_ROOT / "bim" / "module"
+
+
+def _iter_guarded_files():
+    yield from sorted(BIM_MODULE_DIR.glob("*/decorator.py"))
+    yield BIM_MODULE_DIR / "model" / "opening.py"
+
+
+def _is_guarded_class(node: ast.ClassDef) -> bool:
+    return node.name.endswith("Decorator") or node.name == "DecorationsHandler"
+
+
+def _is_mutating_triangulate_call(node: ast.AST) -> bool:
+    if not isinstance(node, ast.Call):
+        return False
+    func = node.func
+    if not isinstance(func, ast.Attribute) or func.attr != "triangulate":
+        return False
+    receiver = func.value
+    if not isinstance(receiver, ast.Attribute) or receiver.attr != "ops":
+        return False
+    inner = receiver.value
+    return isinstance(inner, ast.Name) and inner.id == "bmesh"
+
+
+def test_no_decorator_calls_bmesh_ops_triangulate() -> None:
+    violations: list[str] = []
+    guarded_files = list(_iter_guarded_files())
+    assert guarded_files, "Search root contains no decorator modules — test needs updating."
+
+    for path in guarded_files:
+        try:
+            tree = ast.parse(path.read_text(encoding="utf-8"))
+        except (SyntaxError, FileNotFoundError):
+            continue
+        for class_node in (n for n in ast.walk(tree) if isinstance(n, ast.ClassDef)):
+            if not _is_guarded_class(class_node):
+                continue
+            for sub in ast.walk(class_node):
+                if _is_mutating_triangulate_call(sub):
+                    violations.append(f"{path}:{sub.lineno} {class_node.name} calls bmesh.ops.triangulate")
+
+    assert not violations, (
+        "Decorator classes must not call bmesh.ops.triangulate — it mutates "
+        "the input bmesh and produces fan-clip artefacts at low alpha. "
+        "Use tool.Blender.draw_bmesh_face_tris (wraps bm.calc_loop_triangles). "
+        "Violations:\n  " + "\n  ".join(violations)
+    )