Refresh problem with right aligned rectangle while resizing anNSOutlineView

FROM : malcom
DATE : Wed Nov 28 20:53:30 2007

Hello guys,
I would to place a simple round rectangle (as Mail.app unread badge) at the
right side of an NSOutlineView.
So this is the code I've used to get an NSBezierPath to draw (
http://developer.apple.com/samplecode/Reducer/listing21.html)
while this is the portions of code inside drawInRect event that draw it.

NSBezierPath *roundRect = [NSBezierPath bezierPathWithRoundedRect:
NSMakeRect(NSWidth(cellFrame)-20,inset.origin.y,20,20) cornerRadius: 4.0];
[controlView lockFocus];
[roundRect stroke];
[roundRect fill];
[controlView unlockFocus];

All works fine but when I try to resize the outlinew view the old rectangle
position still visible with the new one
(look here: http://img66.imageshack.us/img66/7857/picture4cf2.png)
the problem does not append if i try to put an NSImage using
compositeToPoint...
Why? Where the code is wrong?
Thanks