Strange drawing behavior of NSSegmentedControl, when superview is drawn

I've got a problem with the NSSegmentedControl. I have a custom view where I draw a gradient with a shadow. The drawing is ok, but when I place a NSSegmentedControl inside it then something strange happens - the background of the custom view is shining through the segmented control, as if the segmented control had a low alpha value.

Here are pictures, that demonstrate the issue :

With the custom view in background :

Without the custom view in background :

As you can see in the left upper corner in the first image the segmented control isn't drawing properly. Here is the drawRect method from the custom view :

- (void)drawRect:(NSRect)dirtyRect {

    NSShadow *shadow = [[NSShadow alloc] init];

    [shadow setShadowOffset:NSMakeSize(0.0, -6.0)];

    [shadow setShadowBlurRadius:3.0];

    [shadow setShadowColor:[[NSColor blackColor] colorWithAlphaComponent:0.3]];

    [shadow set];



    NSGradient *gradient = [[NSGradient alloc] initWithColorsAndLocations:[NSColor colorWithDeviceWhite:0.8 alpha:1.0],(CGFloat)0.0,[NSColor colorWithDeviceWhite:0.65 alpha:1.0],(CGFloat)0.5,[NSColor colorWithDeviceWhite:0.3 alpha:1.0],(CGFloat)0.5,[NSColor colorWithDeviceWhite:0.5 alpha:1.0],(CGFloat)1.0, nil];

    [gradient drawInRect:self.bounds angle:90];



    NSBezierPath *bezierPath = [NSBezierPath bezierPath];

    [bezierPath moveToPoint:NSMakePoint(0, 0)];

    [bezierPath lineToPoint:NSMakePoint(-self.bounds.size.height, 0)];

    [bezierPath lineToPoint:NSMakePoint(-self.bounds.size.height, -self.bounds.size.width)];

    [bezierPath lineToPoint:NSMakePoint(0, -self.bounds.size.width)];



    [[NSColor whiteColor] set];



    [bezierPath stroke];

}    

Can you help me with this? I don't know how to fix this issue.

On 10.7+, Rounded Textured segmented controls and buttons are transparent. On 10.5-10.6, they are gradient filled.