NSButton rendering issues when displayed over NSImageView

Question

I have a NSButton sibling on top of a NSImageView.

Whenever I click the window, there are some rendering issues. It looks like this:

As you can see, the white edges are the problem.
Strangely, this problem even persists if I override drawRect:.
Nothing gets rendered at all, but whenever I click it, those white edges appear.

Also, when the background-image changes, the button gets redrawn and the edges disappear.

Any idea what might cause this?

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EDIT

I found out that this actually happens with every single instance of NSView
and it actually clears part of the buffer (you can see the desktop wallpaper):

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EDIT 2

I also just found out that this does not happen if I layer-back the windows content-view.

Answer 1

Well, this question was impossible for anyone to answer.

My window had a custom contentView, which was just drawing a view with rounded corners.
Instead of using self.bounds, I used dirtyRect to draw the background.

So when the contentView wanted to redraw the background of the controls that were updated, those rounded corners were cut out.