Hello. I found some weird or maybe bugged behavior - scrollbars inside BEFORE preview. It would be easy to ignore them if there were no downsides, but they do have one negative effect - somehow they scale down BEFORE preview image so it looks slightly (1 or maybe even 0.5 pixels...) zoomed out vs AFTER preview and comparison between them becomes wonky.
1) Launch XnConvert 1.99 via command line or launch it normally and add images manually.
2) Use preset and/or some kind of conversion that does not change the image resolution in pixels (PNG to JPEG or some color operation for example).
3) Now you will have these weird scrollbars, but only inside BEFORE preview.

4) You can click twice on Update Preview button to remove scrollbars and make BEFORE and AFTER previews identical for a proper comparison.