See https://raw.github.com/kealist/rebol/master/chip8/layout-bug.r3

load-gui

for i 105 106 1 [
	print i
	img: make image! reduce [to-pair reduce [i i] white]
	repeat num i [poke img to-pair reduce [0 num - 1] red]
	repeat num i [poke img to-pair reduce [i - 1 num - 1] red]
	repeat num i [poke img to-pair reduce [num - 1 0] red]
	repeat num i [poke img to-pair reduce [num - 1 i - 1] red]
	view layout [image img]
] 

This changes if other elements are in the layout. See:

for i 129 130 1 [
	print i
	img: make image! reduce [to-pair reduce [i i] white]
	repeat num i [poke img to-pair reduce [0 num - 1] red]
	repeat num i [poke img to-pair reduce [i - 1 num - 1] red]
	repeat num i [poke img to-pair reduce [num - 1 0] red]
	repeat num i [poke img to-pair reduce [num - 1 i - 1] red]
	view layout [button image img]
]

img: make image! reduce [450x450 white]
repeat num-x 10 [
	repeat num-y 100 [
		poke img to-pair reduce [(439 + num-x - 1) (339 + num-y)] black
	]
]
repeat num-x 10 [
	repeat num-y 100 [
		poke img to-pair reduce [(num-x - 1) (339 + num-y)] black
	]
]
view layout [image img]