Always keep button above the input to prevent input border from disappearing.
Example
<div class="input-group">
<input type="search" placeholder="Hledat" class="form-control" title="Zadejte hledaný termín" />
<span class="input-group-btn">
<button type="submit" class="btn btn-primary btn-raised">Hledat</button>
</span>
</div>