fix(chat): maintain bottom scroll on input resize

react/features/chat/components/web/Chat.js

 
         // Bind event handlers so they are only bound once for every instance.
         this._renderPanelContent = this._renderPanelContent.bind(this);
+
+        // Bind event handlers so they are only bound once for every instance.
+        this._onChatInputResize = this._onChatInputResize.bind(this);
     }
 
     /**
         );
     }
 
+    _onChatInputResize: () => void;
+
+    /**
+     * Callback invoked when {@code ChatInput} changes height. Preserves
+     * displaying the latest message if it is scrolled to.
+     *
+     * @private
+     * @returns {void}
+     */
+    _onChatInputResize() {
+        this._messageContainerRef.current.maybeUpdateBottomScroll();
+    }
+
     /**
      * Returns a React Element for showing chat messages and a form to send new
      * chat messages.
                 <MessageContainer
                     messages = { this.props._messages }
                     ref = { this._messageContainerRef } />
-                <ChatInput />
+                <ChatInput onResize = { this._onChatInputResize } />
             </>
         );
     }

react/features/chat/components/web/ChatInput.js

      */
     dispatch: Dispatch<any>,
 
+    /**
+     * Optional callback to invoke when the chat textarea has auto-resized to
+     * fit overflowing text.
+     */
+    onResize: ?Function,
+
     /**
      * Invoked to obtain translated strings.
      */
                         inputRef = { this._setTextAreaRef }
                         maxRows = { 5 }
                         onChange = { this._onMessageChange }
+                        onHeightChange = { this.props.onResize }
                         onKeyDown = { this._onDetectSubmit }
                         placeholder = { this.props.t('chat.messagebox') }
                         value = { this.state.message } />

react/features/chat/components/web/MessageContainer.js

  * @extends AbstractMessageContainer
  */
 export default class MessageContainer extends AbstractMessageContainer {
+    /**
+     * Whether or not chat has been scrolled to the bottom of the screen. Used
+     * to determine if chat should be scrolled automatically to the bottom when
+     * the {@code ChatInput} resizes.
+     */
+    _isScrolledToBottom: boolean;
+
     /**
      * Reference to the HTML element at the end of the list of displayed chat
      * messages. Used for scrolling to the end of the chat messages.
      */
     _messagesListEndRef: Object;
 
+    /**
+     * A React ref to the HTML element containing all {@code ChatMessageGroup}
+     * instances.
+     */
+    _messageListRef: Object;
+
     /**
      * Initializes a new {@code MessageContainer} instance.
      *
     constructor(props: Props) {
         super(props);
 
+        this._isScrolledToBottom = true;
+
+        this._messageListRef = React.createRef();
         this._messagesListEndRef = React.createRef();
+
+        this._onChatScroll = this._onChatScroll.bind(this);
     }
 
     /**
         });
 
         return (
-            <div id = 'chatconversation'>
+            <div
+                id = 'chatconversation'
+                onScroll = { this._onChatScroll }
+                ref = { this._messageListRef }>
                 { messages }
                 <div ref = { this._messagesListEndRef } />
             </div>
         );
     }
 
+    /**
+     * Scrolls to the bottom again if the instance had previously been scrolled
+     * to the bottom. This method is used when a resize has occurred below the
+     * instance and bottom scroll needs to be maintained.
+     *
+     * @returns {void}
+     */
+    maybeUpdateBottomScroll() {
+        if (this._isScrolledToBottom) {
+            this.scrollToBottom(false);
+        }
+    }
+
     /**
      * Automatically scrolls the displayed chat messages down to the latest.
      *
     }
 
     _getMessagesGroupedBySender: () => Array<Array<Object>>;
+
+    _onChatScroll: () => void;
+
+    /**
+     * Callback invoked to listen to the current scroll location.
+     *
+     * @private
+     * @returns {void}
+     */
+    _onChatScroll() {
+        const element = this._messageListRef.current;
+
+        this._isScrolledToBottom
+            = element.scrollHeight - element.scrollTop === element.clientHeight;
+    }
 }