Notify the timeline it's height has changed, so it can keep being at the bottom

this way new messages will appear in the timeline without needing to scroll

src/components/structures/RoomView.js

@@ -1912,6 +1912,7 @@ export default createReactClass({
                     disabled={this.props.disabled}
                     showApps={this.state.showApps}
                     e2eStatus={this.state.e2eStatus}
+                    resizeNotifier={this.props.resizeNotifier}
                     permalinkCreator={this._getPermalinkCreatorForRoom(this.state.room)}
                 />;
         }

src/components/views/rooms/MessageComposer.js

@@ -354,6 +354,7 @@ export default class MessageComposer extends React.Component {
                     key="controls_input"
                     room={this.props.room}
                     placeholder={this.renderPlaceholderText()}
+                    resizeNotifier={this.props.resizeNotifier}
                     permalinkCreator={this.props.permalinkCreator} />,
                 <UploadButton key="controls_upload" roomId={this.props.room.roomId} />,
                 <EmojiButton key="emoji_button" addEmoji={this.addEmoji} />,

src/components/views/rooms/SendMessageComposer.js

@@ -365,6 +365,13 @@ export default class SendMessageComposer extends React.Component {
     onAction = (payload) => {
         switch (payload.action) {
             case 'reply_to_event':
+                // add a timeout for the reply preview to be rendered, so
+                // that the ScrollPanel listening to the resizeNotifier can
+                // correctly measure it's new height and scroll down to keep
+                // at the bottom if it already is
+                setTimeout(() => {
+                    this.props.resizeNotifier.notifyTimelineHeightChanged();
+                }, 100);
             case Action.FocusComposer:
                 this._editorRef && this._editorRef.focus();
                 break;

src/utils/ResizeNotifier.js

@@ -53,6 +53,10 @@ export default class ResizeNotifier extends EventEmitter {
         this._updateMiddlePanel();
     }
 
+    notifyTimelineHeightChanged() {
+        this._updateMiddlePanel();
+    }
+
     // can be called in quick succession
     notifyWindowResized() {
         // no need to throttle this one,