NSTrackingArea in Today Extension doesn't provide events

Question

I'm working on a Today Extension for the Mac that is supposed to control iTunes. It works and everything, but my UI consists of a square cover art image, overlaid with effect views, which house metadata and controls:

Now, obviously, it'd be nice if I could somehow make those appear only when I needed to—say, when the mouse cursor was above my extension's view.

As I've dealt with this sort of stuff before, I decided to throw together a little NSView subclass that uses NSTrackingArea to fire notifications whenever the mouse enters or exists its bounds:

/**
 * Sets up the tracking area, for the entire bounds of the view.
 */
- (void) setUpTrackingRect {
    _trackingArea = [[NSTrackingArea alloc] initWithRect:self.bounds
                                                 options:NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways | NSTrackingInVisibleRect
                                                   owner:self
                                                userInfo:nil];

    [self addTrackingArea:_trackingArea];
}

/**
 * Mouse entry: send TSMouseTrackingViewMouseEntered notification.
 */
- (void) mouseEntered:(NSEvent *) theEvent {
    NSLog(@"Mouse Enter");
    [[NSNotificationCenter defaultCenter] postNotificationName:TSMouseTrackingViewMouseEntered
                                                        object:self];
}

/**
 * Mouse exit: send TSMouseTrackingViewMouseLeft notification.
 */
- (void) mouseExited:(NSEvent *) theEvent {
    NSLog(@"Mouse Exit");
    [[NSNotificationCenter defaultCenter] postNotificationName:TSMouseTrackingViewMouseLeft
                                                        object:self];
}

(Excerpt from my full code on GitHub.)

Those NSLogs are there for my aid in debugging: they never trigger, despite my cursor moving in and out of the view without issue.

I've been looking through Apple's documentation, and I cannot find anything that explicitly prohibits this sort of thing, or explains why it wouldn't be working. It's a standard NSView subclass, in an NSViewController, but displayed in Notifications Centre, rather than a standalone app.

Any advice as to why this simple tracking area view isn't working in a Today Extension are appreciated.