<div class="container">
    <div class="group in_view">
        <div class="group_name">
            Group 1
        </div>
        <div class="sub_group_name">
            Sub Group Name 1.1
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_name">
            Sub Group Name 1.2
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
    </div>
    <div class="group in_view">
        <div class="group_name">
            Group 2
        </div>
        <div class="sub_group_name">
            Sub Group Name 2.1
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_name">
            Sub Group Name 2.2
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
    </div>
    <div class="group">
        <div class="group_name">
            Group 3
        </div>
        <div class="sub_group_name">
            Sub Group Name 3.1
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_name">
            Sub Group Name 3.2
        </div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
        <div class="sub_group_data">...</div>
    </div>
</div>

.container .group.in_view:first-child .group_name,
.container .group.in_view:only-child .group_name {
    position: fixed;
    top:100px;
}