Make a menu unroll AND open at the same time

Hello,
I have a problem with my theme on this website : https://e-xpertise.ch/demos/mon-3epilier/

As you can see, on the left sidebar there is a navigation menu called "NAVIGATION". And as you can see, most of these menus have a small "+" (plus) symbol on the right.

When we click on this "+" symbol, the sub-menu open (drops down below the main menu element), which is good...

...BUT when we click on one main menu element (for exemple "L'essentiel en bref"), then the page opens, but the sub-menu doesn't drop down as when we clicked on the "+".

In my opinion it's not ergonomic : when a user clicks on a main menu element, sure the page should open, BUT the menu itself should also "open" and display the sub-menu elements, just like when we just click on the "+" symbol.

Is there a way to do that ? I'm sure there is as it's a small thing, but I have no idea how to do that.

Thanks a lot for the continuous great help guys ! :slight_smile: