D105 — Layerless legacy fills inherit the zone's sole layer

Decision

A legacy filled_polygon without a layer inherits the zone's sole copper-mask bit; explicit fill layers always win and zero or multiple copper bits leave the fill layer unknown. The lossless document is not changed.

Alternatives

Drop every layerless fill; choose the first zone layer

Reason

KiCad 5 omits fill-layer tokens for single-layer zones. Dropping them hides real plane copper from obstacle and connectivity checks; guessing on ambiguous zones invents copper.