Third time this has happened. Pretty heavy development session using Arduino IDE 1.8.5 into Teensy 3.6 board.

2013 iMac running High Sierra 10.13.6, 8GB memory.

Symptoms : machine slows down, activity monitor doesn't show anything useful. Teensy serial doesn't respond when attempting to download new build. Shortly followed by black screen of death.

Analytics says that Teensy Serial Monitor may have been the source. It's possible that it ran out of buffer space because I've been using the serial monitor to debug some filter issues. Lots and lots of text.

Here's some screen grabs...

Click image for larger version. 

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Hope this is useful.