Irfanview skinning problem - mysterious white stripe

There is a mysterious white stripe right near the border of irfanview. I tried any windowblinds skin and that whote stripe is there.

I check the background color of the Irfanview and it was dark but that white strip is still there. Does anyone know what the problem is ?

{{EDIT: Does anyone know what it is ?. I've changed all colors in the skin to the black but this white stripe is still there. I guess it is a window background ? }}

#481478