I don't think it has been fixed yet.
Also it seems like there is some weird behavior during the animation where the window/screen (not sure what exactly) is duplicated and it shows for a fraction of a second.
It looks like the duplicated view appears in the foreground and fades-out but in the background the original page seems to be resizing correctly as the viewport size changes.
I also see it on @mazil's video although it's not as heavy as this video I just took (also I think macOS screen capture makes it even more obvious):
Here is a frame of that video showing the superposition of 2 different layouts (one of them is fading-out) during the sidebar animation:

EDIT: I realize this is not exactly about the animation of the sidebar itself, I can create another issue for this if you prefer.