Can I have a line between rows of flexbox?

Question

I have a horizontal navigation, which is somewhat long, and needs to be rearranged for narrow displays. I used flexbox to make it reflow into multiple rows.

But with many rows, the division between the navigation items is not so obvious. I tried giving them a border on top, and it kinda works - but the border is, of course, only visible over the individual navigation options, not creating a nice dividing line between all flexbox rows.

Please view the snippet full page, there is a display problem when it's viewed within the post. Or use this fiddle. You may have to make your browser window narrow to see the navigation in multi row.

header {
    height: 3em;
    background-color: #fff;
}


#main {
    height: 9em;
    background-color: #5987d1;
}

footer {
    height: 3em;
    background-color: #a8a8a8;
    border-top: 1px solid #0047b9;
}

ul.horizontal-nav {
    background: rgba(72, 72, 72, 1);
    display: -moz-box;
    display: -ms-flexbox;
    display: -webkit-flex;
    display: flex;
    -ms-flex-direction: row;
    flex-direction: row;
    align-items: center;
    -ms-flex-wrap: wrap;
    flex-wrap: wrap;
    -webkit-box-shadow: 0px 2px 1px 0px rgba(0,0,0,0.75);
    -moz-box-shadow: 0px 2px 1px 0px rgba(0,0,0,0.75);
    box-shadow: 0px 2px 1px 0px rgba(0,0,0,0.75);
    list-style: none;
}

li.NavigationLi2, li.selected-branch-root {
    padding: 0.75em 1em;
    display: block;
    border-top: 1px solid #2662c3;
}

li.selected-branch-root {
    background: #2662c3;
}

li.NavigationLi2 > a, li.NavigationLi2 > a:visited {
    color: #e6eeff;
    text-decoration: none;
    -ms-word-wrap: nowrap;
    word-wrap: nowrap;
}

li.selected-branch-root > a, li.selected-branch-root > a {
    color: #fff;
    text-decoration: none;
    -ms-word-wrap: nowrap;
    word-wrap: nowrap;
}

<header>
</header>
<nav class="horizontal-nav">
    <ul class="horizontal-nav">
                <li class="selected-branch-root"><a href="/Search">Search</a>
                </li>
                <li class="NavigationLi2"><a href="/My%20models">My models</a>
                </li>
                <li class="NavigationLi2"><a href="/Account">Account</a>
                </li>
                <li class="NavigationLi2"><a href="/Management%20Tools">Management</a>
                </li>
                <li class="NavigationLi2"><a href="/Administration">Administration</a>
                </li>
                <li class="NavigationLi2"><a href="/Help">Help</a>
                </li>
        </ul>
</nav> 
<section id="main">
</section>
<footer>
</footer>

Answer 1

Here is what I do lets say I have a flexbox container with 3 divs inside

<div class="flex">
  <div>
    <h2>Free Shipping</h2>
  </div>
  <div>
    <h2>Everything In Stock</h2>
  </div>
  <div>
    <h2>Largest Inventory</h2>
  </div>
</div>

What I am playing with is in order to make a line right in the middle of the 3 div's / flex items simply just add another flex item between the divs like this:

<div class="flex">
  <div>
    <h2>Free Shipping</h2>
  </div>

  <img src="lib/xxx/img/ydivider.png" alt="divider"/>

  <div>
    <h2>Everything In Stock</h2>
  </div>

  <img src="lib/xxx/img/ydivider.png" alt="divider"/>  

  <div>
    <h2>Largest Inventory</h2>
  </div>
</div>

You can now see we have 5 flex items instead of 3. The 2 additional flex items I find are spaced correctly to be in the middle. Keep in mind if you go to to a breakpoint where you display as columns you will need a horizontal img at that point.

Answer 2

Assuming your philosophy allows you to: 1) Use flebox instead or ul for navigation; and 2) insert tags in your html specifically for aesthetic purposes, then I have a sugestion.

Create a wrapping div to your elements and add to it the CSS you'd like to span the row (background, border, padding...), and add to it the property flex: 1. You might also want to add to it min-width: fit-content (or max-content) to keep it from breaking lines.

I've added below the markup that would show it before and after the quick fix.

#flex-container {
  display: flex;
  flex-wrap: wrap;
}
#flex-container > div {
  padding: 8px 24px;
  border-bottom: 4px solid orange; 
  background: purple; 
}
.flex-menu {
  font-size: 13px;
  font-family: sans;
  color: white;
}
.fix {
  flex: 1;
  min-width: fit-content;
}
.force-break {
  max-width: 340px;
}

<div class="force-break">
  
  <h2>Plain</h2>
    <div id='flex-container'>
      <div class="flex-menu">Search</div>
      <div class="flex-menu">My models</div>
      <div class="flex-menu">Account</div>
      <div class="flex-menu">Management</div>
      <div class="flex-menu">Administration</div>
      <div class="flex-menu">Help</div>
    </div>
  
  <h2>With fix</h2>    
  <div id='flex-container'>
    <div class="fix"><div class="flex-menu">Search</div></div>
    <div class="fix"><div class="flex-menu">My models</div></div>
    <div class="fix"><div class="flex-menu">Account</div></div>
    <div class="fix"><div class="flex-menu">Management</div></div>
    <div class="fix"><div class="flex-menu">Administration</div></div>
    <div class="fix"><div class="flex-menu">Help</div></div>
  </div>
  
</div>