[BUG, MRG] Fix how montage is set in ieeg locate gui (#11421)

doc/changes/latest.inc

@@ -35,6 +35,7 @@ Bugs
 - Fix bug where channel names were not properly sanitized in :func:`mne.write_evokeds` and related functions (:gh:`11399` by `Eric Larson`_)
 - Fix bug where splash screen would not always disappear (:gh:`11398` by `Eric Larson`_)
 - Fix bug where having a different combination of volumes loaded into ``freeview`` caused different affines to be returned by :func:`mne.read_lta` for the same Linear Transform Array (LTA) (:gh:`11402` by `Alex Rockhill`_)
+- Fix how :class:`mne.channels.DigMontage` is set when using :func:`mne.gui.locate_ieeg` so that :func:`mne.Info.get_montage` works and does not return ``None`` (:gh:`11421` by `Alex Rockhill`_)
 
 API changes
 ~~~~~~~~~~~

mne/gui/_ieeg_locate.py

@@ -19,6 +19,7 @@
 
 from matplotlib.colors import LinearSegmentedColormap
 
+from ..channels import make_dig_montage
 from ._core import SliceBrowser
 from ..surface import _voxel_neighbors
 from ..transforms import apply_trans, _get_trans, invert_transform
@@ -84,6 +85,13 @@ def __init__(self, info, trans, aligned_ct, subject=None,
         # load data, apply trans
         self._head_mri_t = _get_trans(trans, 'head', 'mri')[0]
         self._mri_head_t = invert_transform(self._head_mri_t)
+
+        # ensure channel positions in head
+        montage = info.get_montage()
+        if montage and montage.get_positions()['coord_frame'] != 'head':
+            raise RuntimeError('Channel positions in the ``info`` object must '
+                               'be in the "head" coordinate frame.')
+
         # load channels, convert from m to mm
         self._chs = {name: apply_trans(self._head_mri_t, ch['loc'][:3]) * 1000
                      for name, ch in zip(info.ch_names, info['chs'])}
@@ -198,12 +206,14 @@ def _save_ch_coords(self, info=None, verbose=None):
         logger.info('Saving channel positions to `info`')
         if info is None:
             info = self._info
-        with info._unlock():
-            for name, ch in zip(info.ch_names, info['chs']):
-                loc = ch['loc'].copy()
-                loc[:3] = apply_trans(
-                    self._mri_head_t, self._chs[name] / 1000)  # mm->m
-                ch['loc'] = loc
+        montage = info.get_montage()
+        montage_kwargs = montage.get_positions() if montage else \
+            dict(ch_pos=dict(), coord_frame='head')
+        for ch in info['chs']:
+            # surface RAS-> head and mm->m
+            montage_kwargs['ch_pos'][ch['ch_name']] = apply_trans(
+                self._mri_head_t, self._chs[ch['ch_name']].copy() / 1000)
+        info.set_montage(make_dig_montage(**montage_kwargs))
 
     def _plot_ch_images(self):
         img_delta = 0.5

mne/gui/tests/test_ieeg_locate.py

@@ -66,13 +66,25 @@ def test_ieeg_elec_locate_io(renderer_interactive_pyvistaqt):
     import nibabel as nib
     import mne.gui
     info = mne.create_info([], 1000)
-    aligned_ct = nib.MGHImage(np.zeros((256, 256, 256), dtype=np.float32),
-                              np.eye(4))
+
+    # fake as T1 so that aligned
+    aligned_ct = nib.load(op.join(subjects_dir, subject, 'mri', 'brain.mgz'))
+
     trans = mne.transforms.Transform('head', 'mri')
     with pytest.raises(ValueError,
                        match='No channels found in `info` to locate'):
         mne.gui.locate_ieeg(info, trans, aligned_ct, subject, subjects_dir)
 
+    info = mne.create_info(['test'], 1000, 'seeg')
+    montage = mne.channels.make_dig_montage(
+        {'test': [0, 0, 0]}, coord_frame='mri')
+    with pytest.warns(RuntimeWarning, match='nasion not found'):
+        info.set_montage(montage)
+    with pytest.raises(RuntimeError,
+                       match='must be in the "head" coordinate frame'):
+        with pytest.warns(RuntimeWarning, match='`pial` surface not found'):
+            mne.gui.locate_ieeg(info, trans, aligned_ct, subject, subjects_dir)
+
 
 @requires_version('sphinx_gallery')
 @testing.requires_testing_data
@@ -194,4 +206,10 @@ def test_ieeg_elec_locate_display(renderer_interactive_pyvistaqt,
     assert 'mip' in gui._images
     assert 'mip_chs' in gui._images
     assert len(gui._lines_2D) == 1  # LAMY only has one contact
+
+    # check montage
+    montage = raw.get_montage()
+    assert montage is not None
+    assert_allclose(montage.get_positions()['ch_pos']['LAMY 1'],
+                    [0.00726235, 0.01713514, 0.04167233], atol=0.01)
     gui.close()

mne/viz/topomap.py

@@ -1781,7 +1781,7 @@ def plot_evoked_topomap(
         kwargs.update(vlim=_vlim)
         axes.append(plt.subplot(gs[1, :-1]))
         slider = Slider(axes[-1], 'Time', evoked.times[0], evoked.times[-1],
-                        times[0], valfmt='%1.2fs')
+                        valinit=times[0], valfmt='%1.2fs')
         slider.vline.remove()  # remove initial point indicator
         func = _merge_ch_data if merge_channels else lambda x: x
         changed_callback = partial(_slider_changed, ax=axes[0],