What animation?
@BassWrangler That tweet has a "flicker." I can see why, the height of the source tweet image was on the edge of needing the scroll bar. As soon as it draws the bar, it sees the image as small enough to not need the bar and switches back. It then oscillates between having a scroll bar and not having one. The only time I've seen that.
I'm running Firefox on a Win10 machine.