.form-inline with .input-group has wrong positioning when flexbox is enabled
Created by: wojtask9
sample code
<body>
<form class="form-inline">
<div class="form-group">
<div class="input-group">
<span class="input-group-addon" id="basic-addon1">@</span>
<input type="text" class="form-control" placeholder="Username">
</div>
</div>
<div class="form-group">
<div class="input-group">
<div class="input-group-btn">
<button type="button" class="btn btn-secondary dropdown-toggle" data-toggle="dropdown" aria-haspopup="true">
Action
</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="#">Action</a>
</div>
</div>
<input type="text" class="form-control" aria-label="Text input with dropdown button">
</div>
<button type="submit" class="btn btn-primary">Send invitation</button>
</div>
</form>
</body>
screenshoot
If flex is disabled everything looks correct.