When I upgraded to Sierra, my shift key on the external keyboard stopped working when I press a key on my MacBook Pro Retina's keyboard. I have a setup where my left hand is using the external keyboard (to avoid pain due to chest compression), while my right hand is on the MacBook's keyboard.
Now when I press the left shift and a letter on the MacBook keyboard, the letter does not come capitalized. In El Capitan (and all other versions) this worked...
Any hints?
EDIT:
it's not just shift. It's command, alt, shift, control.
the modifier keys do work physically (when used with other keys on the same keyboard).