I just spent an hour-and-a-half with AppleCare and they ware as baffled as I. Every time I connect my LG 5K via its USB-C cable, I lose the ability to use the trackpad or my Bluetooth mouse. Here are the symptoms: 1. Cannot click on menus or select windows. Can only quit applications with Command-Q. Can only right-click on Finder background or any other right-clickable item. 2. After two forced restarts, and after disconnecting from LG 5K, I do regain use of the trackpad. 3. I eliminated any potential log-in item that might be interfering, i.e. TextExpander, ChronoSync, Moom, etc. Did many zap prams, disk repairs, permission repairs, etc. Nothing works. AppleCare advisor had me log in as a guest, did not help. Senior Advisor was equally stumped. Started this Thursday after three weeks of everything working fine. First time it happened I was using Adobe Indesign. I sent Apple a log file. I will let you know what happens.