MM-12422 Post textbox sometimes covers posts at the bottom of the channel (#2115)

* MM-12422 Fix Post textbox sometimes covers posts at the bottom of the channel

components/textbox/textbox.jsx

@@ -16,6 +16,7 @@ import EmoticonProvider from 'components/suggestion/emoticon_provider.jsx';
 import SuggestionBox from 'components/suggestion/suggestion_box.jsx';
 import SuggestionList from 'components/suggestion/suggestion_list.jsx';
 import Constants from 'utils/constants.jsx';
+import {postListScrollChange} from 'actions/global_actions';
 import * as Utils from 'utils/utils.jsx';
 
 const PreReleaseFeatures = Constants.PRE_RELEASE_FEATURES;
@@ -125,6 +126,7 @@ export default class Textbox extends React.Component {
 
     handleHeightChange = (height, maxHeight) => {
         const wrapper = $(this.refs.wrapper);
+        postListScrollChange();
 
         // Move over attachment icon to compensate for the scrollbar
         if (height > maxHeight) {