hotfix(adjoint): correct normal for box minimum face when computing shape gradient

Author: Gregory Roberts

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Fix to `outer_dot` when frequencies stored in the data were not in increasing order. Previously, the result would be provided with re-sorted frequencies, which would not match the order of the original data.
 - Fixed bug where an extra spatial coordinate could appear in `complex_flux` and `ImpedanceCalculator` results.
+- Fixed normal for `Box` shape gradient computation to always point outward from boundary which is needed for correct PEC handling.
 
 ## [2.10.0rc3] - 2025-11-26
 

tidy3d/components/geometry/base.py

@@ -2740,7 +2740,7 @@ def compute_integration_weight(grid_points: NDArray[float]) -> float:
         perps1 = np.zeros_like(grid_points)
         perps2 = np.zeros_like(grid_points)
 
-        normals[:, axis_normal] = 1
+        normals[:, axis_normal] = -1 if (min_max_index == 0) else 1
         perps1[:, axis_perp[0]] = 1
         perps2[:, axis_perp[1]] = 1