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/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,