sub-menus are displayed "inline"

[Mathay]

In Ubuntu 18.04 LTS, the classicmenu indicator displays 'inline'(see attached screenshot)
I mean : when I open a sub-menu, it is not opened in a pop-up parallel window, but it is opened in the same main window along with all the other lines, and this is of course terribly confusing to read.

My installed classicmenu version is 0.10.1

What I do not understand is that when this version was installed in Ubuntu 16, the sub-menus popped up laterally, but now the same version installed in Ubuntu 18 does not pop up but appears inline.
Is there a parameter somewhere to specify this behavior that I could adjust ?
If not, do you have any idea of what happened ?

Thanks for your support,
Lucien.