Yes MC24. Turn off 'DM' Where can I find that in Ubuntu?
Bob means desktop manager/environment, e.g. GNOME in this case. It's something you can't actually turn off, or else your Ubuntu install would only boot to the terminal.
Bob meant a setting within the DM that controls window focus - GNOME doesn't have easy options for stuff like this, and it'd likely would require messing with the dconf-editor. That said, this might be GNOME specific since Ubuntu uses GNOME by default now. I'd recommend waiting to see what Bob finds out here.
KDE Plasma is known to have focus issues with MC, which requires a setting to be changed in Plasma (which is handy, since Plasma has options for this within the settings app for Plasma).
EDIT 1: I see what you guys mean now. That's the character popup that was added in MC, mainly to add Latin characters with diacritics. It's in MC for Windows and Mac too, but it works only if you hold the key down for a second or two.
NEW: Text edit boxes now have a more convenient way to add Latin characters with diacritics. Press and hold a letter key and a list of choices with diacritical versions for that letter is shown to choose from.
The problem is it's triggering by pressing the key (twice in a row) instead of holding down the key (for X amount of time) for it to pop up. I'll test other desktop environments and post the results here.
EDIT 2: Happens in Cinnamon and KDE Plasma too. Can even easily reproduce this in Debian Stretch with GNOME as well. Which means it's likely not desktop environment/distro specific.