Changing preferredContentSize of a navigation controller animates too late

Summary

I'm presenting a UINavigationController as a popover. Each view controller in the nav stack is a UITableViewController, and I want the popover to be the same height as all the rows in each table view controller.

Changing the preferredContentSize of the navigation controller from the -navigationController:willShowViewController:animated: delegate method causes the popover to change size after the push or pop animation is complete, rather than changing with the animation. This is true even if I expressly wrap the assignment in a block that I -animateAlongside... with the navigation controller's transitionCoordinator.

Steps to Reproduce

1. Build and run the attached demo app
2. Tap the Popover button
3. Tap an item in the list that has a different number of rows than the current level

Expected results

Popover resizes while the navigation controller is animating.

Actual results

Popover doesn't begin its resize animation until after the navigation controller has finished animating.

Configuration

iOS 8 Simulator

Version & Build

iOS 8.0 beta 5 (12A4345d)