I'm on 10.13.6. It's not a huge issue, just sort of a pain. Basically, if you're in the Arduino IDE, if you press CMD-U (compile/upload), if you don't CMD-Tab over to the Teensy loader window, you'll sit there for a while until something wakes up and decides to initiate the upload (or finish it, or who knows, since it's hidden from view). As soon as it's visible though, it uploads right away. It's also possible to get yourself into a weird little glitchy situation that requires powering off the Teensy. So let's say you're in this "limbo" state where you've initiated an upload with CMD-U, and then, before it actually uploads you do a CMD-M to see your serial window. I'm not quite sure what happens, but you get an error message that the upload failed, and then it refuses to upload until you've powered down the Teensy and brought it back up.
Like I said, not a huge deal, but you have to get into a habit of Cmd-Tab'ing (or at least I have to... could be just my machine I guess) after an upload to avoid the problem.