Hi
Maybe this isn’t the help you want but I did a quick test of a similar effect for fun here:
http://forum-60215.webflow.io
The links has a fixed width and I moving the border div inside of a link wrapper with IX2 and percentages.
You also need to think about page transitions. For a user when a link to another page is clicked the new page is gonna load if you don’t stop it.
So one approach would be to use a script thats delays the page transition, make your animation and then the page transition happen. On the new page the link is marked as the user would expect.
Maybe some other have other tricks or tips for you =)