PLT-6890 Fix various scrolling issues (#6727)

* Fix various scrolling issues

* Move reaction scrolling to reaction list

* Handle scrolling when RHS opens

* Only run scroll update code when posts change

webapp/utils/constants.jsx

@@ -1,7 +1,7 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See License.txt for license information.
 
-import keyMirror from 'key-mirror/keyMirror.js';
+import keyMirror from 'key-mirror';
 
 import audioIcon from 'images/icons/audio.png';
 import videoIcon from 'images/icons/video.png';

webapp/utils/event_types.jsx

@@ -0,0 +1,8 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import keyMirror from 'key-mirror';
+
+export default keyMirror({
+    POST_LIST_SCROLL_CHANGE: null
+});

webapp/utils/global_event_emitter.jsx

@@ -0,0 +1,27 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+import AppDispatcher from 'dispatcher/app_dispatcher.jsx';
+import EventEmitter from 'events';
+
+import EventTypes from 'utils/event_types.jsx';
+
+class GlobalEventEmitterClass extends EventEmitter {
+    constructor() {
+        super();
+        this.dispatchToken = AppDispatcher.register(this.handleEventPayload);
+    }
+
+    handleEventPayload = (payload) => {
+        const {type, value, ...args} = payload.action; //eslint-disable-line no-use-before-define
+
+        switch (type) {
+        case EventTypes.POST_LIST_SCROLL_CHANGE:
+            this.emit(type, value, args);
+            break;
+        }
+    }
+}
+
+const GlobalEventEmitter = new GlobalEventEmitterClass();
+export default GlobalEventEmitter;

webapp/utils/markdown.jsx

@@ -4,11 +4,11 @@
 import * as TextFormatting from './text_formatting.jsx';
 import * as SyntaxHighlighting from './syntax_highlighting.jsx';
 
+import {postListScrollChange} from 'actions/global_actions.jsx';
+
 import marked from 'marked';
 import katex from 'katex';
 
-import ScrollStore from 'stores/scroll_store.jsx';
-
 function markdownImageLoaded(image) {
     if (image.hasAttribute('height') && image.attributes.height.value !== 'auto') {
         const maxHeight = parseInt(global.getComputedStyle(image).maxHeight, 10);
@@ -22,7 +22,8 @@ function markdownImageLoaded(image) {
     } else {
         image.style.height = 'auto';
     }
-    ScrollStore.emitPostScroll();
+
+    postListScrollChange();
 }
 global.markdownImageLoaded = markdownImageLoaded;