Fix AMR swarm migration IndexError after mesh.adapt() (#135)

src/underworld3/discretisation/discretisation_mesh.py

@@ -1653,6 +1653,16 @@ def nuke_coords_and_rebuild(
         # Invalidate projected boundary normals (rebuilt lazily on access)
         self._projected_normals = None
 
+        # BUGFIX(#135): invalidate the per-cell face control-point arrays.
+        # These are populated lazily by _get_mesh_face_control_points, sized
+        # (num_faces, num_local_cells, dim). After mesh.adapt() the new mesh
+        # has a different cell count, so the stale arrays from the old mesh
+        # would be indexed with new-mesh cell IDs in
+        # _test_if_points_in_cells_internal — producing IndexError when the
+        # new cell count exceeds the old one (and silent corruption otherwise).
+        self.faces_outer_control_points = None
+        self.faces_inner_control_points = None
+
         # BUGFIX(#130): refill the coord cache for every already-registered
         # variable. Variables created before this rebuild would otherwise
         # have their cache entry (from __init__) wiped above and refill

tests/test_0810_amr_swarm_migration_regression.py

@@ -0,0 +1,58 @@
+"""Regression test for issue #135.
+
+After mesh.adapt(), swarm migration into the new mesh raised IndexError in
+Mesh._test_if_points_in_cells_internal because the per-cell face control-point
+caches (faces_outer_control_points / faces_inner_control_points) were not
+invalidated by nuke_coords_and_rebuild. Cell IDs from the new mesh would index
+into stale arrays sized for the old mesh's cell count.
+
+Reproducer reduced from the user's report (bknight1, 2026-04-23). Skipped if
+the running PETSc was not built with the pragmatic backend.
+"""
+
+import pytest
+import numpy as np
+import sympy
+
+import underworld3 as uw
+from test_0800_optional_modules import requires_amr
+
+
+pytestmark = [pytest.mark.level_1, requires_amr]
+
+
+@requires_amr
+def test_swarm_migration_after_adapt_does_not_raise():
+    """mesh.adapt() then swarm._force_migration_after_mesh_change() must not
+    raise IndexError on the failing case from issue #135.
+    """
+    mesh = uw.meshing.UnstructuredSimplexBox(
+        minCoords=(-1.0, 0.0),
+        maxCoords=(1.0, 1.0),
+        cellSize=0.2,
+        regular=False,
+        qdegree=3,
+    )
+
+    swarm = uw.swarm.Swarm(mesh=mesh)
+    swarm.populate(fill_param=4)
+
+    cells_before = mesh.dm.getHeightStratum(0)[1]
+
+    x, h_min, h_max, sigma = sympy.symbols("x h_min h_max sigma", positive=True)
+    h_expr = h_min + (h_max - h_min) * (1 - sympy.exp(-((x / sigma) ** 2)))
+    h_subbed = h_expr.subs({h_min: 0.05, h_max: 0.5, sigma: 0.25})
+    h_vals = uw.function.evaluate(h_subbed, mesh.X.coords)
+
+    metric = uw.adaptivity.create_metric(mesh, h_vals)
+    mesh.adapt(metric)
+
+    cells_after = mesh.dm.getHeightStratum(0)[1]
+    assert cells_after != cells_before, (
+        "test premise: mesh.adapt should change cell count for this metric"
+    )
+
+    # Pre-fix: this line raises
+    #   IndexError: index N is out of bounds for axis 1 with size M
+    # because faces_*_control_points kept the old (cells_before-sized) arrays.
+    swarm._force_migration_after_mesh_change()