What's this button on Touch Bar?

[evanavevan]

I'm not really sure what I did, but it just started showing up and I can't seem to get rid of it. When I press it, it basically hides any App Controls to the left that are specific to the current app being used. It's kind of useless so I'd like to get rid of it, since it makes the other Control Strip buttons smaller. Anyone seen this?

 

[deadworlds]

That button usually shows up for me when I'm viewing some kind of video in safari. Pressing it brings up a scrub bar to change my position in the video. Strange that yours seems to be hiding controls.

 

[justywusty]

It's the Now Playing button. Music or video playing and it will appear so you can quickly access player controls.

 

[monkeydax]

Adds a scrub bar to all your currently open videos. Check out this review of the touchbar (scrub bar section at 5:28 of the video the link):