Layer-Backed view no longer punches hole in transparent NSWindow - cocoa

Question

I am trying to make a "hole" in an NSWindow using a CAShapeLayer or even just a CALayer.
When using regular NSViews or even layer-backed views, I can override drawRect: using code like this:
[spotImage drawInRect:self.bounds fromRect:NSZeroRect operation:NSCompositeXOR fraction:1.0];
where spotImage is an NSImage with pure white content and some gradations, and the window has a black background with 0.5 alpha. The NSView subclass where this drawRect is defined has a clearColor background.
The end result is a grey window (It is a transparent window with a styleMask of NSBorderlessWindowMask as can be found in many samples.
If I turn the NSView into a layer-backed view, it calls the drawRect methods and works fine.
When I turn this into a layer-hosting view, and again use the same structure (NSWindow > contentView > CustomView) then, the drawInRect method just draws the image. It no longer punches a hole through it.
It is like the layer itself can no longer punch the hole when it is part of a layer-hosting hierarchy.
Here is some sample code:
The custom NSWindow subclass initializer:
- (id)initWithContentRect:(NSRect)contentRect styleMask:(NSUInteger)aStyle backing:(NSBackingStoreType)bufferingType defer:(BOOL)flag {
self = [super initWithContentRect:contentRect styleMask:NSBorderlessWindowMask backing:NSBackingStoreBuffered defer:NO];
if (self) {
[self setBackgroundColor: [NSColor lightGrayColor]]; //[NSColor clearColor]];
[self setAlphaValue:0.5];
[self setOpaque:NO];
[self setHasShadow: NO];
[self useOptimizedDrawing:YES];
[self setIgnoresMouseEvents:YES];
}
return self;
}
the code in my applicationDidFinishLaunching method:
PPContentView *thisView = [[PPContentView alloc]
initWithFrame:CGRectInset([self.window.contentView bounds], 50, 50)];
//[thisView setWantsLayer:YES]; enabling this makes things opaque again
[self.window.contentView addSubview:thisView];
thisView.layer.backgroundColor = [NSColor clearColor].CGColor;
//Create custom content
[thisView setNeedsDisplay:YES];
}
and my custom view's drawRect contains:
[[NSImage imageNamed:#"spotFuzzy.png"] drawInRect:self.bounds fromRect:NSZeroRect operation:NSCompositeXOR fraction:1.0];