UI rendering issues with Segmented Control

I'm working on a simple app to learn Cocoa and I've encountered a little rendering bug with the segmented controller.

As you can see here, the view does not display fully opaque. I've looked a开发者_运维问答round to try and find a solution to this to no avail. I did at one point think that maybe this control wasn't supposed to be used in this way but a quick browse through the system preferences yielded several views using this technique.

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The control you are looking for is actually NSTabView. Docs here:

NSTabView