Flex-end alignment with scrolling

Solution to http://stackoverflow.com/questions/36130760/use-justify-content-flex-end-and-to-have-vertical-scrollbar

HTML

<div class="session-textchat">
  <div class="past-messages">
    <div class="receiver">
      <span class="message">
        Good afternoon David. Welcome to your appointment! How are you today?
      </span>
    </div>
    <div class="sender">
      <span class="message">
        Hello doctor. I feel terrible to be honest.
      </span>
    </div>
    <div class="receiver">
      <span class="message">
        I can see from your notes that you've been having some ear ache - can you tell me a bit more about your symptoms?
      </span>
    </div>
    <div class="sender">
      <span class="message">
        Hello doctor. I feel terrible to be honest.
      </span>
    </div>
    <div class="receiver">
      <span class="message">
        I can see from your notes that you've been having some ear ache - can you tell me a bit more about your symptoms?
      </span>
    </div>
  </div>
</div>

CSS

.session-textchat {
    height: 520px;
    background: #fff;
}
.session-textchat .past-messages {
    width: 100%;
    max-width: 980px;
    height: 100%;
    margin: 0 auto;
    overflow-y: auto;
    display: -webkit-flex;
    display: flex;
    -webkit-flex-direction: column-reverse;
    flex-direction: column-reverse; 
    align-items: flex-end;
}
.session-textchat .past-messages > :first-child {
    margin-top: auto !important;
}
.session-textchat .past-messages .receiver,
.session-textchat .past-messages .sender {
    flex: 0 0 47px;
    margin: 0 0 10px;
}
.session-textchat .past-messages .receiver .message,
.session-textchat .past-messages .sender .message {
    display: inline-block;
    padding: 17px;
    -moz-border-radius: 4px;
    -webkit-border-radius: 4px;
    border-radius: 4px; 
}
.session-textchat .past-messages .receiver {
    -webkit-align-self: flex-start;
    align-self: flex-start; 
    background: #f4f4f4;
    color: #535353; 
}
.session-textchat .past-messages .sender {
    -webkit-align-self: flex-end;
    align-self: flex-end; 
    background: url('../img/rgbapng/0050ff26.png');
    background: rgba(0, 80, 255, 0.15);
    color: #0050ff; 
}