Delete region on B/W images is shifted

[chameleon]

How to reproduce:

- Set background color as COLOR1 (white)
- Create a completely COLOR2 (black) 1 bit image (monochrome).
- Select a region
- Press key delete

What I expect: The selected region becomes COLOR1 (white)

What happens: An equal size region, a few pixels righter than selected region, becomes COLOR1 (white).

This happens only in 1 bit images.

Something wrong with bit padding on end of each row?

[xnview]

i can't reproduce, how do you create 1 bit image in Xnview?