A hairline of background colour drawn inside each tile's own outline, following the shape rather than its bounding box. It is what keeps a run of twenty passing jobs from merging into one green rectangle at zero gap, and on a honeycomb it reads as the wall between cells. Two pixels suits most screens; go up to four or five on a 4K panel viewed from far away. Zero removes it, at the cost of no longer being able to count tiles of the same colour.