impossible to type an option dash on mc command line

when using the mc shell (i.e the blue panel are up and you are typing on the bottom 'command line', the '-' key pops up the selection window instead of entering a dash. This makes it impossible to type any commands with an option in this state, which is an enormous usability problem.

Using ctrl-O to hide the panels means that you can now type a '-', but of course you can't insert a path of filename with the ctrl-x-t or ctrl-x-p keycombos.

I have no idea what is going wrong here, but it's _really_ annoying! I think normally only the keypad '-' key has this 'unselect' meaning? If I run the debian version of mc in a stable chroot on this machine then the same problem arises, strongly suggesting that this is something in the input layer, not the actual binary or config being run.

I'll check this on a real debian box to work out how it normally works when I have one to hand.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: mc 3:4.8.15-2
ProcVersionSignature: Ubuntu 4.13.0-39.44~16.04.1-generic 4.13.16
Uname: Linux 4.13.0-39-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CurrentDesktop: XFCE
Date: Mon May 14 23:43:56 2018
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
SourcePackage: mc
UpgradeStatus: No upgrade log present (probably fresh install)