Sierra: Dock Menu disappears too fast
There are two ways of navigating menus on macOS:


Modern: Click or right-click and release the button, then click on an option by once again depressing the button and releasing it again.
Classic: Click or right-click and keep holding the button, then move the mouse to the option, where releasing the button on an option selects the option.


It seems like macOS is receiving inputs following half of each of these: the click is held long enough that macOS thinks you're going to use the classic option, but you're actually trying to use modern.

There are a couple of possibilities for why this is occurring:


There's a bug with the Dock. Restart the Dock with killall Dock or log out and back in.
The mouse is broken and registering a longer click than it should be. Is the button sticking down? Do you have an alternative mouse to try?