Two cells by two, not one — the inner rules have nowhere to show on a single cell, and they are half of what a border edit is about. The lines are a real StaticBorderRenderer fed the selection's own strokes, so what you click is what the grid will draw.
Aiming is by DISTANCE rather than by zone (see edgesAt), and the feedback is the edge itself: hovering lights the line a click would toggle, along its whole length — both inner rules, over their crossing. Shading a quadrant says "somewhere around here"; a lit line says which line.
Excel's Format Cells → Border diagram: a block of cells whose edges you click to draw or clear.