Why does this textured NSWindow suddenly change its background gradient when resizing?

Can someone please tell me, why the background gradient of the textured NSWindow in this app suddenly changes, when you make the window a little bit 开发者_高级运维smaller?

This is the minimal example I could find, that exhibits this behaviour. App & Source are available via Dropbox.

-- Updates:

1. If you put the slider lower,the gradient does not change when resizing the window:

   

2. Also, the change seems to happen when the distance between the slider and the window's right border gets smaller than the HIG says it should be.

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It is really interesting question =)
I don't shure, but guess, this problem is connected to layers displaying.

If you still want to use textured window, you can put additional NSView object in the interface builder between NSView and NSSlider (NSWindow -> NSView -> NSView -> NSSlider). It fixes the bug.