verdi says we should use the photon animation curve and 250ms duration.

FWIW, left, width and padding transitions are not be able to run on the compositor, which means it consumes the main-thread to update styles on every tick. Also it causes reflow.

(In reply to Hiroyuki Ikezoe (:hiro) from comment #3)

FWIW, left, width and padding transitions are not be able to run on the compositor, which means it consumes the main-thread to update styles on every tick. Also it causes reflow.

It's a positioned element so the reflow should be limited to that? Happy to look into other ways to implement this animation though, if you have suggestions.

https://hg.mozilla.org/mozilla-central/rev/48e712ae6367

(In reply to Dão Gottwald [::dao] from comment #4)

(In reply to Hiroyuki Ikezoe (:hiro) from comment #3)

FWIW, left, width and padding transitions are not be able to run on the compositor, which means it consumes the main-thread to update styles on every tick. Also it causes reflow.

It's a positioned element so the reflow should be limited to that? Happy to look into other ways to implement this animation though, if you have suggestions.

If we don't expect siblings of the transitioning element is not moved by the transition, left and padding transition can be replaced by transform. I have no idea about the width transition. Unfortunately transform transition with geometric change property transition, e.g. width, left, etc. can't be run on the compositor.

I can confirm the animation is implemented. I verified using Fx 74.0a1(2020-02-04) on Windows 10 x64, Ubuntu 18.04 and macOS 10.15.
Updating the flags accordingly.