iframes don't resize fast enough

[sole]

On this demo: https://mozdevs.github.io/servo-experiments/experiments/bookshelf/

the iframes that contain each book content are meant to use 100% of the width of their containers. However when the transition is started, the iframes don't resize fast enough to resize simultaneously with their parent as you can see here:
https://youtu.be/Voa0BburMIo?t=12s

It seems there are two ways of reproducing this:

1. using a slowish laptop like my MacBook retina, launching with ./mach run -w -r ... (although the issue is slightly less terrible when running Servo on a non-retina external monitor)
2. using a faster laptop like a MacBook Pro, using Servo+Browser.html, or Nightly

Reproduced using a "freshly compiled" Servo from sources on this MacBook

Of note: if I try to run this demo without WebRender all I get is a grey screen with the dog and the footer:

[jdm]

cc @glennw for the webrender bustage.

[paulrouget]

[nox]

Nothing even resizes anymore for me.