The box's TOP edge in PDF-native bottom-left coordinates, measured up from the page
bottom.
Same conversion as pdfYFlip, taken at the other edge:
pdfYFlip(h, box).y === pdfTopEdgeY(h, box) - box.height. Drawing that hangs off the
top of a field — a text baseline inset from the top, a mark centred in the box — is
expressed from this edge, so the conversion is never re-derived at the call site.
The box's TOP edge in PDF-native bottom-left coordinates, measured up from the page bottom.
Same conversion as pdfYFlip, taken at the other edge:
pdfYFlip(h, box).y === pdfTopEdgeY(h, box) - box.height. Drawing that hangs off the top of a field — a text baseline inset from the top, a mark centred in the box — is expressed from this edge, so the conversion is never re-derived at the call site.