[Solved] KeypressEvent for Modifier keys delayed on Linux, Windows
TL;DR: shift and other modifier key press events were being delayed.
I am almost certain this is due to testing in a virtual machine under Parallels. Parallels VM supports a number of its own special key shortcuts. I strongly suspect that the VM supervisor is sitting on the modifier key press until it knows for sure that I am not trying to use one of its special shortcuts. Then it delivers everything to the virtual system.