getting the stupid compose key to work again (gnome-flashback-metacity, upgrade to 16.04)

[Adam Funk]

I recently upgraded a machine to 16.04 (a bit late, I know) & I can't
get the compose key to work (lwin is my preference, but nothing
works).

I've tried dconf-editor, dconf CLI, & finally gsettings; they all say
'compose:lwin', but it still doesn't work.

$ gsettings get org.gnome.desktop.input-sources xkb-options
['compose:lwin']

(I'm using gnome-flashback-metacity, BTW.)

I'd be grateful for any further suggestions.

Thanks,
Adam