[huxiaoyi-ovo]

Summary

FlowField assumes every cell has valid neighbors in the 3×3 neighborhood. On unpadded maps, both integration-field propagation and direction assignment can index cells outside the grid and raise KeyError.

It also attempts to choose a navigation direction for goal-disconnected regions even though all reachable-cost values there are infinite. These cells should not point toward an arbitrary invalid neighbor.

Fix

• During integration propagation, visit only cells already registered in integration_field (the traversable cells within map bounds).
• During vector assignment, consider only valid, finite-cost neighbors.
• Assign (None, None) to cells disconnected from the goal, allowing path following to terminate rather than follow an undefined direction.
• Prevent the animation loop from attempting to plot the (None, None) termination marker after reaching the goal or encountering an unreachable cell.
• Preserve the existing cost model, search propagation, and public interface.

Regression tests

Added four focused cases:

1. An unpadded 3×3 free grid with the goal at a corner. Every generated direction must lead to the goal without revisiting a cell.
2. A 4×4 grid separated by a solid wall. An unreachable start has infinite integration cost and no navigation direction.
3. A one-cell map whose start equals the goal.
4. An animation-enabled two-cell path that confirms the terminal sentinel is not passed to Matplotlib.

Scope: two files; no new dependencies.

Validation: The four focused edge cases pass in an isolated local reproduction of the updated FlowField methods (4/4). Full cross-platform repository CI is still pending: the fork validation PR #11 has not produced Actions runs yet, and the upstream checks require maintainer action.

AI disclosure: ChatGPT assisted with code analysis, the fix, and regression tests.