{# One panel: its ground, then a fixed box at its place on the side.
The ground is the panel's colour run into the bleed on every trim edge, so
a cut that lands a hair outside the trim leaves no white sliver (#188). It
is a sibling of the box rather than its child, because the box clips.
The copy sits `inset` from every edge -- horizontally through the frame's
pad_x, which every container already applies, vertically through the
box's own padding -- and is clipped at the safe line rather than painting
into its neighbour. The empty span closing the copy is the overflow
sentinel: the print engine records where it lands. Anchored lower (#355),
the copy and its sentinel sit in one cell the copy's height, set by CSS
`vertical-align`: WeasyPrint ignores the `valign` attribute, and a height
on the table rather than the cell. The sentinel is a block there, so it
opens no line box and lands on the copy's last line. An overfull cell
grows downward, so the sentinel still lands past the safe line. A panel
holding a bled section (#396) grows its box into the bleed on each trim
edge and pads the same back, so the copy keeps its place and the box clips
past the trim rather than at it; the copy's own box then clips nothing. #}