Full Screen Cocoa Application - Force Quit - cocoa

Question

I am facing a weird issue when trying to make a full screen application. I am currently using a a NSWindow with a NSBorderlessWindowMask to make the screen show at top, and I subclassed NSWindow to accept keys:
- (BOOL) canBecomeKeyWindow
{
return YES;
}
This works fine, and I can use keys in my window, and quit out of the application with Command + Q. However, when I try to force quit the application on my Mac, the screen freezes, and I must restart my computer. I have a simple WebView inside my Window:
mainWindow = [[MyBorderWindow alloc] initWithContentRect:screenRect
styleMask:NSBorderlessWindowMask
backing:NSBackingStoreBuffered
defer:NO screen:[NSScreen mainScreen]];
[mainWindow setLevel:windowLevel];
[mainWindow setBackgroundColor:[NSColor blackColor]];
[mainWindow makeKeyAndOrderFront:nil];
NSView *contentView = [mainWindow contentView];
WebView *customView = [[WebView alloc] initWithFrame:[contentView bounds]];
[customView setTranslatesAutoresizingMaskIntoConstraints:NO];
[contentView addSubview:customView];
I have tried the application on another computer, and force quit does not freeze the screen. Both computers are running OS X Mavericks.
Please let me know if there is anything else I should add.
Thanks in advance,
Bucco
Edit:
I think that my applicationWillTerminate method in AppDelegate might be part of the issue, but am not sure. Why would this method work fine on another computer, but not mine?
- (void)applicationWillTerminate:(NSNotification *)notification
{
[mainWindow orderOut:self];
// Release the display(s)
if (CGDisplayRelease( kCGDirectMainDisplay ) != kCGErrorSuccess) {
NSLog( #"Couldn't release the display(s)!" );
}
}