Grid Tile diameter = 1

A setting of 1 for diameter on a grid tile will also recognize adjacent tiles as walls.
Why is this the case when setting it to 0.8, it only recognizes its own tile?


Due to floating point errors and other imprecisions in the physics system, this can happen.
You can expand the visualization in the grid graph settings to show the sphere/capsule that the grid graph uses to query the physics system.