Cannot animate fade-in of NSView using animator proxy (r.11801145)

AppDelegate.m

@implementation AppDelegate

@synthesize window = _window;
@synthesize box = _box;
@synthesize addButton = _addButton;
@synthesize useDummyAnimationGroup = _useDummyAnimationGroup;

- (void)_prepareForFadeIn;
{
    [_box setAlphaValue:0];
    [_box setHidden:NO];
}

- (void)addBoxToWindow:(id)sender;
{
    // Both of these branches should have the same effect: the view should fade in smoothly.
    // On 10.7, it only works the first time if _useDummyAnimationGroup==YES. After that, it works regardless.
    // On 10.8, the fade-in never works the first time. Subsequent fade-ins work.
    if (_useDummyAnimationGroup) {
        [NSAnimationContext runAnimationGroup:^(NSAnimationContext *context) {
            context.duration = 0;
            
            [self _prepareForFadeIn];
            
        } completionHandler:nil];
    } else {
        
        [self _prepareForFadeIn];
        
    }
    
    [NSAnimationContext runAnimationGroup:^(NSAnimationContext *context) {
        context.duration = 1;
        [_box.animator setAlphaValue:1];
    } completionHandler:^{
        NSLog(@"Fade in complete!");
    }];
}

- (void)removeBoxFromWindow:(id)sender;
{
    [NSAnimationContext runAnimationGroup:^(NSAnimationContext *context) {
        context.duration = 1;
        [_box.animator setAlphaValue:0];
    } completionHandler:^{
        [_box setHidden:YES];
        NSLog(@"Fade out complete!");
    }];
}

@end