Scrollable div to stick to bottom, when outer div changes in size

Here is an example chat app ->

The idea here is to have the .messages-container take up as much of the screen as it can. Within .messages-container, .scroll holds the list of messages, and in case there are more messages then the size of the screen, scrolls.

Now, consider this case:

  1. The user scrolls to the bottom of the conversation
  2. The .text-input, dynamically gets bigger

Now, instead of the user staying scrolled to the bottom of the conversation, the text-input increases, and they no longer see the bottom.

One way to fix it, if we are using react, calculate the height of text-input, and if anything changes, let .messages-container know

componentDidUpdate() {
  window.setTimeout(_ => {
    const newHeight = this.calcHeight();
    if (newHeight !== this._oldHeight) {
      this.props.onResize();
    }
    this._oldHeight = newHeight;
  });
}

But, this causes visible performance issues, and it’s sad to be passing messages around like this.

Is there a better way? Could I use css in such a way, to express that when .text-input-increases, I want to essentially shift up all of .messages-container

Answer

2:nd revision of this answer

Your friend here is flex-direction: column-reverse; which does all you ask while align the messages at the bottom of the message container, just like for example Skype and many other chat apps do.

.chat-window{
  display:flex;
  flex-direction:column;
  height:100%;
}
.chat-messages{
  flex: 1;
  height:100%;
  overflow: auto;
  display: flex;
  flex-direction: column-reverse;
}

.chat-input { border-top: 1px solid #999; padding: 20px 5px }
.chat-input-text { width: 60%; min-height: 40px; max-width: 60%; }

The downside with flex-direction: column-reverse; is a bug in IE/Edge/Firefox, where the scrollbar doesn’t show, which your can read more about here: Flexbox column-reverse and overflow in Firefox/IE

The upside is you have ~ 90% browser support on mobile/tablets and ~ 65% for desktop, and counting as the bug gets fixed, …and there is a workaround.

// scroll to bottom
function updateScroll(el){
  el.scrollTop = el.scrollHeight;
}
// only shift-up if at bottom
function scrollAtBottom(el){
  return (el.scrollTop + 5 >= (el.scrollHeight - el.offsetHeight));
}

In the below code snippet I’ve added the 2 functions from above, to make IE/Edge/Firefox behave in the same way flex-direction: column-reverse; does.

function addContent () {
  var msgdiv = document.getElementById('messages');
  var msgtxt = document.getElementById('inputs');
  var atbottom = scrollAtBottom(msgdiv);

  if (msgtxt.value.length > 0) {
    msgdiv.innerHTML += msgtxt.value + '<br/>';
    msgtxt.value = "";
  } else {
    msgdiv.innerHTML += 'Long long content ' + (tempCounter++) + '!<br/>';
  }
  
  /* if at bottom and is IE/Edge/Firefox */
  if (atbottom && (!isWebkit || isEdge)) {
    updateScroll(msgdiv);
  }
}

function resizeInput () {
  var msgdiv = document.getElementById('messages');
  var msgtxt = document.getElementById('inputs');
  var atbottom = scrollAtBottom(msgdiv);

  if (msgtxt.style.height == '120px') {
    msgtxt.style.height = 'auto';
  } else {
    msgtxt.style.height = '120px';
  }
  
  /* if at bottom and is IE/Edge/Firefox */
  if (atbottom && (!isWebkit || isEdge)) {
    updateScroll(msgdiv);
  }
}


/* fix for IE/Edge/Firefox */
var isWebkit = ('WebkitAppearance' in document.documentElement.style);
var isEdge = ('-ms-accelerator' in document.documentElement.style);
var tempCounter = 6;

function updateScroll(el){
  el.scrollTop = el.scrollHeight;
}
function scrollAtBottom(el){
  return (el.scrollTop + 5 >= (el.scrollHeight - el.offsetHeight));
}
html, body { height:100%; margin:0; padding:0; }

.chat-window{
  display:flex;
  flex-direction:column;
  height:100%;
}
.chat-messages{
  flex: 1;
  height:100%;
  overflow: auto;
  display: flex;
  flex-direction: column-reverse;
}

.chat-input { border-top: 1px solid #999; padding: 20px 5px }
.chat-input-text { width: 60%; min-height: 40px; max-width: 60%; }


/* temp. buttons for demo */
button { width: 12%; height: 44px; margin-left: 5%; vertical-align: top; }

/* begin - fix for hidden scrollbar in IE/Edge/Firefox */
.chat-messages-text{ overflow: auto; }
@media screen and (-webkit-min-device-pixel-ratio:0) {
  .chat-messages-text{ overflow: visible; }
  /*  reset Edge as it identifies itself as webkit  */
  @supports (-ms-accelerator:true) { .chat-messages-text{ overflow: auto; } }
}
/* hide resize FF */
@-moz-document url-prefix() { .chat-input-text { resize: none } }
/* end - fix for hidden scrollbar in IE/Edge/Firefox */
<div class="chat-window">
  <div class="chat-messages">
    <div class="chat-messages-text" id="messages">
      Long long content 1!<br/>
      Long long content 2!<br/>
      Long long content 3!<br/>
      Long long content 4!<br/>
      Long long content 5!<br/>
    </div>
  </div>
  <div class="chat-input">
    <textarea class="chat-input-text" placeholder="Type your message here..." id="inputs"></textarea>
    <button onclick="addContent();">Add msg</button>
    <button onclick="resizeInput();">Resize input</button>
  </div>
</div>

Side note 1: The detection method is not fully tested, but it should work on newer browsers.

Side note 2: Attach a resize event handler for the chat-input might be more efficient then calling the updateScroll function.

Note: Credits to HaZardouS for reusing his html structure