bootstrap hover moving all the links - html

Question

I'm playing around with bootstrap and I have some problem. I created a navigation with few links and I want bottom border to appear when I hover the link. I made it and now, when I hover, links are "moving" to the top, even though I didn't give them any margin or padding...
Here is JSFiddle with the example, you need to resize the window of outcome to be able to see entire menu.
https://jsfiddle.net/zhzmz6fa/
<div class="collapse navbar-collapse" id="navbarTogglerDemo02">
<ul class="navbar-nav ml-auto mt-2 mt-lg-0">
<li class="nav-item active">
<a class="nav-link" href="#">Home<span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">O nas</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Opis</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Kontakt</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Archiwum</a>
</li>
</ul>
</div>
.navbar {
font-family: sans-serif;
background-color: orangered;
}
.navbar-toggler {
border: none;
}
.nav-item {
padding-left: 1.5rem;
}
.nav-link:hover,
.nav-link:active {
border-bottom: 2px solid black;
transition: border-bottom .5s;
}

Answer 1

Borders count extra in the calculation of width. So if you have
width: 50px;
border-right: 1px solid black;
The element will be 51px wide.
This is also what happens with your code. The solution would be to always give it a border, but have it transparent.
.nav-link {
border-bottom: 2px solid transparent;
}

Answer 2

An alternative to border is a pseudoelement.
Style it as you need, set it's opacity to 0. On :hover, or :active change the opacity to 1.
.navbar {
font-family: sans-serif;
background-color: orangered;
}
.navbar-toggler {
border: none;
}
.nav-item {
padding-left: 1.5rem;
}
.nav-link {
position: relative;
}
.nav-link:after {
content: '';
position: absolute;
left: 0;
right: 0;
display: block;
height: 2px;
opacity: 0;
background: black;
}
.nav-link:hover:after,
.nav-link:active:after {
opacity: 1;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
<nav class="navbar navbar-expand-lg navbar-light fixed-top">
<img src="http://via.placeholder.com/250x100" class="navbar-brand">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarTogglerDemo02" aria-controls="navbarTogglerDemo02" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarTogglerDemo02">
<ul class="navbar-nav ml-auto mt-2 mt-lg-0">
<li class="nav-item active">
<a class="nav-link" href="#">Home<span class="sr-only">(current)</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">O nas</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Opis</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Kontakt</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Archiwum</a>
</li>
</ul>
</div>
</nav>