Pause button started working for Chrome / Youtube, messing with audio player controls I'm on macbook pro, 2017, Mojave 10.14.4. 

Previously, I used F7/F8/F9 audio-buttons with Spotify only (it worked with iTunes if it's open together with Spotify but whatever. It didn't do anything in Chrome).

I don't remember exactly when, but recently it started working with youtube videos in Chrome, and the sad thing is that if both Spotify and a youtube window are open, it changes from Spotify to Youtube. (e.g. start playing a song in Spotify, open a youtube link, and it starts working for youtube only). This is very confusing, can I somehow change this behavior?
This is a new feature of Chrome73, called Hardware Media Key Handling in chrome.
Enter this URL in the address bar then you can simply disable it.
 chrome://flags/#hardware-media-key-handling