fix bug when inserting snapping points near sim min boundary

Author: dmarek-flex

CHANGELOG.md

@@ -20,6 +20,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Fixed bug in broadband adjoint source creation when forward simulation had a pulse amplitude greater than 1 or a nonzero pulse phase.
 - Fixed shaping of `CustomMedium` gradients when permittivity data includes a frequency dimension with multiple entries.
+- Bug when generating a grid with snapping points near the simulation boundaries.
 
 ### Changed
 - Relaxed bounds checking of path integrals during `WavePort` validation.

tests/test_components/test_grid_spec.py

@@ -83,7 +83,7 @@ def test_make_coords_with_snapping_points():
     # 3) snapping takes no effect if it's too close to interval boundaries
     # forming the simulation boundary
     coord = gs.grid_x.make_coords(
-        snapping_points=((0.98, 0, 0),),
+        snapping_points=((0.98, 0, 0), (-0.98, 0, 0)),
         **make_coords_args,
     )
     assert np.allclose(coord_original, coord)

tidy3d/components/grid/mesher.py

@@ -160,7 +160,7 @@ def insert_snapping_points(
             # give priority to the snapping_point and replace
             elif d_1 < min_step:
                 # Don't replace if the boundary is the simulation boundary
-                if ind == 0:
+                if ind == 1:
                     continue
                 interval_coords[ind - 1] = new_coord
                 continue