Track left panel width using ResizeObserver

src/@types/global.d.ts

@@ -43,6 +43,7 @@ import TypingStore from "../stores/TypingStore";
 import { EventIndexPeg } from "../indexing/EventIndexPeg";
 import {VoiceRecordingStore} from "../stores/VoiceRecordingStore";
 import PerformanceMonitor from "../performance";
+import UIStore from "../stores/UIStore";
 
 declare global {
     interface Window {
@@ -82,6 +83,7 @@ declare global {
         mxEventIndexPeg: EventIndexPeg;
         mxPerformanceMonitor: PerformanceMonitor;
         mxPerformanceEntryNames: any;
+        mxUIStore: UIStore;
     }
 
     interface Document {