I'm looking to get drop-downs from the navbar menu. I've installed webcouturier.dropdownmenu, which seemed to be the way to go for that. I added it to the eggs, ran the buildout, went into Site Settings > Add-Ons and installed it, went into Site Settings > Add-On Settings > Dropdown Menus and looked over the settings, verified that they all seem good, saved them, etc. During this process I didn't encounter any errors or other unexpected behaviour, but it doesn't seem to do anything.
Worth noting is that the function to disable clicking on anything with children, from its control panel, DOES work. If I turn it on, I can't click on any navbar folder that has children. If I disable it, the behaviour reverts to normal. However, nothing else happens -- no drop-down menus whether I hover over the menus, click on them, etc.
Doesn't help you with plone 5.1.6 , but there is a very nice dropdownmenu implementation now in core Plone 5.2. Maybe the time on getting a drop down menu solution working in Plone 5.1.6 is better spent on migration the project to 5.2 if there are no other blockers preventing you from doing the upgrade.
Oh, that's good to know! I actually had originally deployed 5.2, but our infrastructure person forced us back versions -- originally to 5.0.8, but then after arguing with him for some time we compromised on 5.1.6. (He apparently doesn't like to run the most recent version of anything, on the argument that it's not tested enough in the wild yet. sigh) Anyway, I can bring this in as an argument in favour of 5.2.
I did install the webcouturier add-in on a 5.1.5 test site that I hadn't gotten around to deleting yet and it worked well there. I also created a new 5.1.6 site on our 5.1.6 dev server and didn't modify it at all except to install the add-in (following the same steps) and it didn't work. So perhaps there's a 5.1.6-specific bug.