Definitely one of the most complex animations yet: window resizing.
Just the crossfade effect itself took a while to get working with all the window geometries and buffer offsets, and then there's the whole multiple window orchestration with Wayland's asynchronous nature. (I don't do animation transactions yet, that'll be a whole other level of complexity on top.)
Happy with the result though, and it's cool that it seamlessly works with block-out-from screencast.