I went to try to sync my iPhone in the music app again and tried this full-screen window again and for some reason the sync button is now covered up by a tab menu and inaccessible! So, I went into my display settings (under system settings) and adjusted to the smallest text option under the “built-in-display” setting. I then re-opened the music tab, clicked on my phone , selected full-screen mode, and the sync button was uncovered and clickable again.
So that worked to both uncover the sync button and make it responsive to a mouse click. It would be nice if Apple would fix this glitch in the next update. Also, maybe add another command key function to allow us to go into the iPhone sync tab, or an alternate route as my iPhone never shows up in the finder “devices” list anymore, it only shows up in music and photos.