fix[adjoint]: correct scaling for adjoint sources created from field monitors

Author: Gregory Roberts

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - Compatibility with `xarray>=2025.03`.
 - Inaccurate gradient when auto-grabbing permittivities for structures using `td.PolySlab` when using dispersive material models.
+- Fixed scaling for adjoint sources when differentiating with respect to `FieldData` to account for the mesh size of the monitor and thus the created source. This aligns adjoint gradient magnitudes with numerical finite difference gradients for field data.
 
 ## [2.8.1] - 2025-03-20
 

pyproject.toml

@@ -275,7 +275,10 @@ ignore = [
 
 [tool.pytest.ini_options]
 # TODO: remove --assert=plain when https://github.com/scipy/scipy/issues/22236 is resolved
-addopts = "--cov=tidy3d --doctest-modules -n auto --dist worksteal --assert=plain"
+addopts = "--cov=tidy3d --doctest-modules -n auto --dist worksteal --assert=plain -m 'not numerical' "
+markers = [
+    "numerical: marks numerical tests for adjoint gradients that require running simulations (deselect with '-m \"not numerical\"')",
+]
 env = ["MPLBACKEND=Agg", "OMP_NUM_THREADS=1"]
 doctest_optionflags = "NORMALIZE_WHITESPACE ELLIPSIS"
 norecursedirs = [

tests/conftest.py

@@ -1,6 +1,8 @@
 import os
+from pathlib import Path
 
 import autograd
+import matplotlib as mpl
 import matplotlib.pyplot as plt
 import numpy as np
 import psutil
@@ -68,3 +70,33 @@ def pytest_xdist_auto_num_workers(config):
     if os.getenv("GITHUB_ACTIONS"):
         return cores
     return max(1, cores - 1)
+
+
+@pytest.fixture
+def mpl_config_noninteractive():
+    """Configure matplotlib non-interactive backend for all tests in this module."""
+    original_backend = mpl.get_backend()
+    mpl.use("Agg")
+    yield
+    plt.close("all")
+    mpl.use(original_backend)
+
+
+@pytest.fixture
+def mpl_config_interactive():
+    """Configure matplotlib interactive backend for all tests in this module."""
+    original_backend = mpl.get_backend()
+    mpl.use("TkAgg")
+    yield
+    mpl.use(original_backend)
+
+
+@pytest.fixture
+def dir_name(request):
+    return request.param
+
+
+@pytest.fixture
+def create_directory(dir_name):
+    if dir_name is not None:
+        directory = Path(dir_name).mkdir(parents=True, exist_ok=True)