With High Sierra, Apple changed how the media playback control keys work on Macs without Touch Bars. In the past, these keys were locked to iTunes, even if media was playing in Safari, Messages or another application. Now, if I view an Animoji video from someone in Messages, the media keys become disassociated with iTunes, and they seem to get stuck with Messages far too long.
This has been driving me crazy on my iMac, where I keep iTunes running nearly all day. Thankfully, Milan Toth has fixed this regression with a little free Mac app named High Sierra Media Key Enabler. I’ve been running it since I saw Brian Stucki tweet about it, and it’s pretty great.