I've noticed that for a few days now, on my computer running Mojave 10.14.6, that if I hold any of the modifier keys down, I lose the ability to left or right click, preventing me from using any application that uses the modifier+click combo or selecting multiple items in a list that aren't right next to each other
I use a Corsair K70 Mk.2 keyboard and Corsair M65 mouse with the ckb-next driver, the Magic Trackpad doesn't work for this either.