Cover the ancestor clash and name what drawCheck compares by

drawfence.go

@@ -226,7 +226,8 @@ func newDrawWalk() *drawWalk {
 
 // walk follows what rendering n renders. A repeat renders over draws of its own, so the walk stops
 // there. A cell whose row the pins keep out has no case: a pinned table renders its pinned row
-// alone, so that cell never renders on this route.
+// alone and an unpinned one draws a row inside its nearest pinned ancestor's, so that cell never
+// renders on this route.
 func (w *drawWalk) walk(n node, at drawAt) {
 	v := nodeVisit{n, at.group, at.rowsKey()}
 	if w.seen[v] {