I'm not sure what Aero does but it's probably alpha blending which would follow a different paradigm. Two alphas overlayed should result in transparent lines. It could be that when the length or width of the selection is zero the code does not draw anything. This does make sense since no selection should equal no selection rect.On Vista, it creates a solid translucent blue rectangle. If I try to make a straight line, I get no line or dots at all.
This could happen depending on how the code deals with this condition. It may begin the line slightly above the old line which could result in the pattern you are seeing. Code it up in a Win32 app and play with it to see what happens. I'm sure you can duplicate the behavior.hat are you saying? I can clearly see two dots.
On Windows XP I do get two dots. But it looks like an overlayed XOR line to me.