FIX: correct spacing attribute in SourceMorph when src_to is provided (#13687)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Famous077

doc/changes/dev/13687.bugfix.rst

@@ -0,0 +1 @@
+Fix bug in :func:`~mne.compute_source_morph` where :attr:`mne.SourceMorph.spacing` was set incorrectly when ``src_to`` is provided, by `Famous077`_.

mne/morph.py

@@ -292,8 +292,11 @@ def compute_source_morph(
             assert morph_mat.shape[0] == n_verts
 
     vertices_to = vertices_to_surf + vertices_to_vol
+
     if src_to is not None:
         assert len(vertices_to) == len(src_to)
+        # set spacing to None when src_to is provided
+        spacing = None
     morph = SourceMorph(
         subject_from,
         subject_to,

mne/tests/test_morph.py

@@ -308,6 +308,17 @@ def test_surface_vector_source_morph(tmp_path):
     source_morph_surf = compute_source_morph(
         inverse_operator_surf["src"], subjects_dir=subjects_dir, smooth=1, warn=False
     )  # smooth 1 for speed
+    src_to_fs = mne.read_source_spaces(
+        subjects_dir / "fsaverage" / "bem" / "fsaverage-ico-5-src.fif"
+    )
+    morph_with_src_to = compute_source_morph(
+        inverse_operator_surf["src"],
+        subjects_dir=subjects_dir,
+        src_to=src_to_fs,
+        smooth=1,
+        warn=False,
+    )
+    assert morph_with_src_to.spacing is None
     assert source_morph_surf.subject_from == "sample"
     assert source_morph_surf.subject_to == "fsaverage"
     assert source_morph_surf.kind == "surface"