Fix set_icon_gizmo_position so billboard ignores object rotation

set_icon_gizmo_position computed
``mw @ (Translation @ billboard_rot @ Scale)`` — the object's world
matrix was applied AFTER the billboard rotation, so any non-trivial
object rotation (e.g. a wall rotated in plan, a stair rotated to
match a corridor) carried over into the icon's transform and tilted
it edge-on to the camera instead of facing it.

Switch to ``billboarded_at(world_pos, billboard_rot, scale)`` where
``world_pos = mw @ local_pos``: translate to world space first, then
apply the billboard rotation independently of the object's rotation.
This matches the manual pattern the base class's
``update_editing_gizmos`` already uses for validate/cancel/cycle for
exactly this reason.

Drops the now-stale workaround docstring on
``GizmoWallEdition._update_icon_row_extras`` that documented why it
bypassed ``set_icon_gizmo_position`` — the helper does the right
thing now.

Adds ``test/bim/module/model/test_stair_gizmos.py`` as the regression
guard: parametrised over six rotation angles, asserts that the rotation
part of the resulting matrix equals ``billboard_rot`` (no contribution
from ``mw``'s rotation) and that the translation lands at
``world_pos``. Also exercises ``set_icon_gizmo_position`` end-to-end via
a stub gizmo to catch the exact shape of the previously-broken call
site.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/drawing/gizmos.py

@@ -4847,8 +4847,8 @@ class BaseParametricGizmoGroup:
             scale: Gizmo scale factor (default 0.5)
         """
         if gz := self.get_gizmo_if_visible(gizmo_name):
-            local_transform = Matrix.Translation(Vector((x, y, z))) @ billboard_rot @ Matrix.Scale(scale, 4)
-            gz.matrix_basis = mw @ local_transform
+            world_pos = mw @ Vector((x, y, z))
+            gz.matrix_basis = billboarded_at(world_pos, billboard_rot, scale)
 
     def set_dimension_gizmo_position(
         self,

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

@@ -1969,13 +1969,8 @@ class GizmoWallEdition(bpy.types.GizmoGroup, gizmo.BaseParametricGizmoGroup):
         - Toggle-openings icon next to the pen. Lives outside edit mode because
           opening visibility is a viewport-display concern, not a wall-edit action.
 
-        Uses the manual ``Translation(world_pos) @ billboard_rot @ Scale`` pattern
-        rather than the base class's ``set_icon_gizmo_position`` helper. The helper
-        computes ``mw @ (Translation @ billboard_rot @ Scale)``, which applies the
-        wall's rotation to the billboard — for a wall rotated in plan, the icons
-        end up tilted edge-on to the camera instead of facing it. The base class's
-        own ``update_editing_gizmos`` already uses the manual pattern for validate/
-        cancel/cycle for exactly this reason; we match it here."""
+        Uses ``billboarded_at`` directly for parity with the base class's
+        ``update_editing_gizmos`` validate/cancel/cycle pattern."""
         if not hasattr(self, "rotate_gizmo"):
             return
         gizmo_prefs = self.get_gizmo_prefs()

src/bonsai/test/bim/module/model/test_stair_gizmos.py

@@ -0,0 +1,128 @@
+# 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.
+
+"""Regression guard for the stair icon billboard fix.
+
+Before the fix, ``set_icon_gizmo_position`` in ``bim.module.drawing.gizmos``
+composed ``mw @ (Translation @ billboard_rot @ Scale)``, which applied the
+stair's world rotation on top of the billboard rotation. The result was
+icons (validate / cancel / lock / +/- / cycle / tread_lock) drawn edge-on
+to the camera for any stair rotated in plan — effectively unclickable.
+
+The fix routes through ``billboarded_at(world_pos, billboard_rot, scale)``,
+which computes ``Translation(world_pos) @ billboard_rot @ Scale`` — the
+object's rotation is folded into the translation only, never the rotation."""
+
+import math
+import types
+
+import bpy
+import pytest
+from mathutils import Matrix, Vector
+
+pytestmark = pytest.mark.model
+
+
+@pytest.fixture(autouse=True)
+def _require_real_bpy():
+    if not isinstance(bpy, types.ModuleType) or hasattr(bpy, "_mock_name"):
+        pytest.skip("requires real Blender (bpy is mocked or absent)")
+
+
+def _rotation_close(a: Matrix, b: Matrix, tol: float = 1e-6) -> bool:
+    for row_a, row_b in zip(a, b):
+        for va, vb in zip(row_a, row_b):
+            if abs(va - vb) > tol:
+                return False
+    return True
+
+
+@pytest.mark.parametrize("angle_deg", [0, 30, 45, 90, 135, 217])
+def test_billboarded_at_rotation_is_pure_billboard(angle_deg):
+    """Object rotation must not leak into the gizmo's rotation part."""
+    from bonsai.bim.module.drawing.gizmos import billboarded_at
+
+    mw = Matrix.Rotation(math.radians(angle_deg), 4, "Z") @ Matrix.Translation((3, 4, 5))
+    billboard_rot = Matrix.Rotation(math.radians(30), 4, "X")
+
+    world_pos = mw @ Vector((1, 0, 2))
+    result = billboarded_at(world_pos, billboard_rot, scale=0.5)
+
+    # The rotation part of result, after stripping the 0.5 uniform scale,
+    # must equal billboard_rot — no contribution from mw's rotation.
+    rotation_part = result.to_3x3() * 2.0
+    assert _rotation_close(rotation_part.to_4x4(), billboard_rot)
+
+
+def test_billboarded_at_translation_is_world_pos():
+    """Translation lands exactly at the world-space target."""
+    from bonsai.bim.module.drawing.gizmos import billboarded_at
+
+    world_pos = Vector((1.23, 4.56, 7.89))
+    result = billboarded_at(world_pos, Matrix.Identity(4), scale=0.5)
+    assert (result.translation - world_pos).length < 1e-6
+
+
+def test_set_icon_gizmo_position_does_not_apply_object_rotation():
+    """End-to-end: the helper used by every stair icon (and shared with all
+    parametric gizmo groups) must produce a matrix whose rotation part is
+    billboard_rot, not mw_rotation @ billboard_rot. This is the exact bug
+    that left stair icons edge-on to the camera."""
+    from bonsai.bim.module.drawing.gizmos import (
+        BaseParametricGizmoGroup,
+        billboarded_at,
+    )
+
+    # Same inputs as the real call site (stair.py:747-765), but we drive the
+    # helper directly so we don't need a registered GizmoGroup. We bind a
+    # stand-in `get_gizmo_if_visible` that returns a tiny mock; the helper's
+    # observable output is the matrix_basis it assigns.
+    captured = {}
+
+    class _GizmoStub:
+        matrix_basis: Matrix = Matrix.Identity(4)
+
+    stub = _GizmoStub()
+
+    def _fake_get(name):
+        captured["name"] = name
+        return stub
+
+    # Bind the helper to a throwaway instance so `self.get_gizmo_if_visible`
+    # resolves to our stub without registering a real GizmoGroup with Blender.
+    fake_self = types.SimpleNamespace(get_gizmo_if_visible=_fake_get)
+    BaseParametricGizmoGroup.set_icon_gizmo_position(
+        fake_self,
+        "validate_gizmo",
+        mw=Matrix.Rotation(math.radians(45), 4, "Z") @ Matrix.Translation((3, 4, 5)),
+        x=1.0,
+        y=0.0,
+        z=2.0,
+        billboard_rot=Matrix.Rotation(math.radians(30), 4, "X"),
+        scale=0.5,
+    )
+
+    expected_world_pos = (Matrix.Rotation(math.radians(45), 4, "Z") @ Matrix.Translation((3, 4, 5))) @ Vector((1, 0, 2))
+    expected = billboarded_at(expected_world_pos, Matrix.Rotation(math.radians(30), 4, "X"), 0.5)
+
+    assert captured["name"] == "validate_gizmo"
+    for row_a, row_b in zip(stub.matrix_basis, expected):
+        for va, vb in zip(row_a, row_b):
+            assert abs(va - vb) < 1e-6