Canvas updates itself only on mouseup

[kpudlik]

http://jsfiddle.net/5KKQ2/2464/

I want black rectangle to fill the gap between red rectangle and top canvas border. It works when I move red rect:

But when I try to achieve the same behaviour for resizing then it looks messy. Or to be more specific the correct result is showed only after mouseup. During mousedown/mousemove calculations are broken.

Isn't it some kind of internal bug?