Animating Auto Layout changes concurrently with NSPopover contentSize change

I'm attempting to reproduce the iTunes 11 behavior of navigable views within a popover. I can't seem to find a way to get my animation to happen at the same time as the popover's contentSize change happens, though.

The basic setup I have is a custom view subclass MyPopoverNavigationView with two subviews: the old and new views that I want the popover to navigate between. The popover's contentViewController has a MyPopoverNavigationView instance as its view. I do this:

// Configure constraints how I want them to show the new popover view
[NSAnimationContext runAnimationGroup:^(NSAnimationContext *ctx) {
    [ctx setDuration:0.25];
    [ctx setAllowsImplicitAnimation:YES];
    [self layoutSubtreeIfNeeded];
} completionHandler:nil];

As far as I can tell from the Auto Layout WWDC 2012 videos, this is the recommended way to animate changes to views' frames as a result of constraint changes. It works, but the animation happens in two phases:

• First, the popover's contentSize will change to accommodate the new view that I'm moving to (before that view becomes visible, so it partially obscures the existing content).
• Second, the views animate as I expect, so that the constraints system I installed is satisfied.

From setting some breakpoints, it looks like -layoutSubtreeIfNeeded eventually calls a private method on the popover called _fromConstraintsSetWindowFrame:, which does the popover size animation outside my animation group. My context's duration isn't respected, and my animations don't happen until the popover's size change is complete.

How can I get my views to animate together with the popover's size change?

Turns out the trick is to explicitly set the popover's contentSize property outside of the animation and completion blocks. The relevant snippet from the sample GitHub project I put together to figure it out looks like:

// Configure constraints for post-navigation view layout
[NSAnimationContext runAnimationGroup:^(NSAnimationContext *ctx) {
    [ctx setDuration:0.25];
    [ctx setAllowsImplicitAnimation:YES];
    [self layoutSubtreeIfNeeded];
} completionHandler:^{
    // Tear down some leftover constraints from before the transition
}];

// Explicitly set popover's contentSize so its animation happens simultaneously
containingPopover.contentSize = postTransitionView.frame.size;