ReadonlybackThe back wall's two edges: LINES, 4 segments. null whenever footprint is.
ReadonlyfaceThe cut face: TRIANGLE_STRIP, 6 vertices (2 columns × 3 rows), faded to nothing at both ends.
ReadonlyfaceThe face's two side edges: LINES, 4 segments (each rail split at mid-height to carry the fade).
ReadonlyfootprintGround outline of the cut volume: LINE_LOOP, 4 vertices. null for a near-horizontal cut.
The gizmo's geometry, in metres relative to the slice's anchor (plane-local ENU). Pure output — built and unit-tested without a GL context.