Some clipping occur at some characters when draw_white_space is all. Is this inteded or a bug?

as seen in http://img220.imageshack.us/img220/721/screenshot20110202at653.png the "@" characted is clipped while i draw_white_space setting is all and font is Monaco. This does not happens when i am using Courier New. Yet it can be observer in some other fonts (i tried "Helvetica Neue" but it is not monospace etc.). So is this an intended result or a bug?