At one point in macOS history, if you had a Play/Pause button on your keyboard, it kept “focus” on whatever the last music app you used was. Then Apple broadened that to include anything with a music player. You could be in iTunes and use the keyboard to pause playback, then switch to YouTube in a browser, press play, and the keyboard would help you play and pause there, too.
Or at least most of the time. In my experience and that of many people who post about it, the last music player isn’t always tracked well. That’s especially the case if you close a window in a browser that had a video or audio player in it, and then press the Play/Pause button.
The problem predates Mojave, and Catalina doesn’t improve on it. But if you regularly use iTunes or Spotify or switch between them, a simple and free utility will at least let you lock play/pause focus to one or both of those apps.
Mac Media Key Forwarder 3.1 works with Catalina, and creates a simple menu that you can select behavior from, such as choosing iTunes instead of Spotify or vice versa. The app will no longer be developed, but the 3.1 version works and is downloadable from this file-transfer site operated by its developer.
For a somewhat more complicated but more exhaustive media-playback-focus controller across both audio and video, try BeardedSpice, which is also free.
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