When using both console and serial, with input via serial, if one tries to scroll quickly through a submenu using the down arrow key they will be brought back to the main menu as if escape had been pressed. The assumption is that somehow the escape sequence for "down arrow" on serial is being partially lost and instead interpreted as just escape.

Steps to reproduce as a bash script:

#!/bin/bash

mkdir -p /tmp/overlay/boot/grub/

cat << "EOF" > /tmp/overlay/boot/grub/grub.cfg
serial
terminal_output console serial
terminal_input console serial

submenu "Long submenu" {

  for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20; do

  menuentry "Menu entry #${i}" {
    true
  }
  done
}
EOF

grub-mkrescue -o /tmp/test.iso /tmp/overlay/
kvm -serial stdio /tmp/test.iso

# Use the terminal window for input, rather than the qemu window,
# Select "Long submenu" then scroll to the bottom of the submenu