fix: bug in local field projection for far_field_approx=True

Author: yaugenst-flex

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Regression in local field projection leading to incorrect results for `far_field_approx=True`.
+
 ## [2.7.6] - 2024-10-30
 
 ### Added

tidy3d/components/field_projection.py

@@ -482,8 +482,11 @@ def contract(currents):
 
         order = [idx_u, idx_v, idx_w]
         zeros = np.zeros(jm[0].shape)
-        J = anp.array([*jm[:2], zeros])[order]
-        M = anp.array([*jm[2:], zeros])[order]
+
+        # for each index (0, 1, 2), if it’s in the first two elements of order,
+        # select the corresponding jm element for J or the offset element (+2) for M
+        J = anp.array([jm[order.index(i)] if i in order[:2] else zeros for i in range(3)])
+        M = anp.array([jm[order.index(i) + 2] if i in order[:2] else zeros for i in range(3)])
 
         cos_theta_cos_phi = cos_theta[:, None] * cos_phi[None, :]
         cos_theta_sin_phi = cos_theta[:, None] * sin_phi[None, :]