The + and - of the quantity box (cart) lays over the menu

[lmeyer1]

On the cart page, when opening the menu, the + and - of the quantity box remain visible. In the screenshot, the menu is semi-transparent to allow to see what is underneath:

The quantity box's style comes directly from bootstrap:
bootstrap/scss/_input-group.scss, line 62 says about it :

// Prepend and append
//
// While it requires one extra layer of HTML for each, dedicated prepend and
// append elements allow us to 1) be less clever, 2) simplify our selectors, and
// 3) support HTML5 form validation.

Therefore, we need to increase z-index in the menu.