> \boxXX=
\vbox(20.0+0.0)x300.0
% colsep
.\vbox(0.0+0.0)x300.0
..\hbox(20.0+0.0)x300.0
...\glue 50.0
...\hbox(20.0+0.0)x100.0
....\glue 100.0
...\glue 150.0
% rowsep
.\hbox(0.0+0.0)x300.0
..\glue 50.0
..\glue 100.0
..\glue 150.0
% cell
.\hbox(20.0+0.0)x300.0
..\glue 50.0
..\hbox(20.0+0.0)x100.0
...\vbox(20.0+0.0)x100.0, glue set 20.0fil
....\glue 0.0
....\glue 0.0 plus -1.0
....\glue 0.0 plus 1.0fil
....\glue 0.0
..\glue 150.0
.\penalty 10000
% rowsep
.\hbox(0.0+0.0)x300.0
..\glue 50.0
..\glue 100.0
..\glue 150.0