Animations should stop until the next turn of the event loop if script reads layout properties

[pcwalton]

roc pointed out a potential web-compat problem in off-main-thread animations. This could fail in Servo, whereas it succeeds in all other browser engines:

let el = ... some element ... ;
let r0 = el.getBoundingClientRect();
let r1 = el.getBoundingClientRect();
assert(r0.left == r1.left);

This is because the animation refresh could tick in between let r0 = ... and let r1 = ... and mess with the layout.

The easiest way to fix this is to stop all animations until the next turn of the event loop if script performs layout queries.

[nox]

@pcwalton Wasn't this fixed?

[jdm]

I do not believe we have any code that stops animations from progressing if script performs a layout query.