As mentioned in #2139, try the following:

function postexec --on-event fish_postexec
    echo postexec
end
function prompt --on-event fish_prompt
    echo prompt
end
sleep 5 # Press ctrl-c to abort

Neither event will fire. When the sleep runs to completion both will.

This completely precludes a fishscript solution to #2139 because every command could disable the keypad again (i.e. be equivalent to tput rmkx; sleep 5)