The case of an X11 server having multiple displays is handled correctly by the
code in src/mainx.c. However, due to some functions not being correctly
refactored and still getting the first screen (and also the first root window)
from the XCB connection, i3 was operating on the wrong root window.
Apparantly, after activating numlock once, the numlock modifier
stays turned on (use xev(1) to verify). So, to resolve useful
keysyms, we remove the numlock flag from the event state. What
currently does not work is actually using your keypad.
Michael Stapelberg