I found plenty of help for centering a login form but once centered it's slightly offset by what I believe must be the navbar. I can't figure out why this is happening as the centering relies on the body height set at 100% in my css. How do I center the login form in the center of the document without it being offset by the navbar? (it's lower than the exact center of the page) What I mean is to the eye the form isn't in the center of the screen.
CSS
html, body {
background-color: lightgray;
height: 100%;
PHP/HTML
<div class="container h-100">
<div class="row h-100 justify-content-center align-items-center">
<div class="col-10 col-md-8 col-lg-6">
<form action="{{ path('login') }}" method="post" >
<div class="form-group">
<label for="_username">Username:</label>
<input type="text" class="form-control" placeholder="Username..." required name="_username">
</div>
<div class="form-group">
<label for="_password">Password:</label>
<input type="password" class="form-control" placeholder="Password..." required name="_password">
</div>
<div class="form-group">
<button class="btn btn-success">Login</button>
</div>
</form>
</div>