If the slider bar is showing up on your phone, it should now show up when voting from your computer as well.

It's not a new feature, but you need a certain amount of VESTS or Steem Power before the slider bar will show up.

Last I heard, you need about 1 million VESTS before the slider bar will appear. Currently, you have 711 Steem Power which is equivalent to 1.48 million VESTS. You can see that on the right side of the screen here: https://steemdb.com/@naiahyoung

You may have broke the 1M VESTS threshold when your last post paid out.