Keep on-boundary 3D points inside the domain (fix corner/edge/face NaN masking)

[aabills]

Description

UnstructuredSubMesh.contains_points_3d decides whether 3D query points lie inside the mesh domain, using the generalized winding number (sum of signed solid angles over the boundary triangles), and returns winding > 2 * np.pi.

A point sitting exactly on the boundary surface gets a partial solid angle that lands at or below the halfway threshold, so it is misclassified as outside:

point location	winding	> 2π?
interior	4π	✅ inside
flat face	2π	❌ (not strictly >) → outside
edge	π	❌ → outside
corner	π/2	❌ → outside
exterior	0	❌ outside

contains_points_3d is called (via ProcessedVariable._get_boundary_mask → _interpolate_spatial, and via get_3d_slices) to blank out-of-domain points to NaN when interpolating or plotting a 3D unstructured field. So any query point that lands exactly on a domain face/edge/corner — which happens routinely when a regular plotting/sampling grid touches the domain boundary — is masked to NaN. The solution itself is unaffected; this is post-processing/plotting only.

Verified empirically on a unit cube: winding is exactly π/2 at a corner, π on an edge, 2π on a face, 4π in the interior, and 0 outside — matching the geometry.

Fix

Lower the threshold below the smallest on-surface value (π/2 at a corner), so interior and on-boundary points count as inside while true exterior (winding ≈ 0) stays outside:

return winding > 0.1

Testing

• New test test_contains_points_3d_boundary_points_are_inside asserts a corner, edge midpoint, face centre, and interior point are all inside, and an exterior point is not. It fails before the change (corner/edge/face classified outside) and passes after — verified by reverting the source line alone.
• uv run --group dev pytest packages/pybamm/tests/unit/test_meshes — 163 passed.
• uv run pre-commit run --all-files clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.11%. Comparing base (f82976f) to head (ccd225c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5704   +/-   ##
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  Coverage   98.11%   98.11%           
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  Files         340      340           
  Lines       32744    32744           
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  Hits        32128    32128           
  Misses        616      616           

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[aabills]

Closing: out of scope for #5688. Pre-existing item in already-merged #5687 mesh code, not part of the spatial-method PR.