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 {

src/components/structures/LeftPanel.tsx

@@ -67,6 +67,7 @@ const cssClasses = [
 
 @replaceableComponent("structures.LeftPanel")
 export default class LeftPanel extends React.Component<IProps, IState> {
+    private ref: React.RefObject<HTMLDivElement> = createRef();
     private listContainerRef: React.RefObject<HTMLDivElement> = createRef();
     private groupFilterPanelWatcherRef: string;
     private bgImageWatcherRef: string;
@@ -93,6 +94,10 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         });
     }
 
+    public componentDidMount() {
+        UIStore.instance.trackElementDimensions("LeftPanel", this.ref.current);
+    }
+
     public componentWillUnmount() {
         SettingsStore.unwatchSetting(this.groupFilterPanelWatcherRef);
         SettingsStore.unwatchSetting(this.bgImageWatcherRef);
@@ -100,6 +105,7 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         RoomListStore.instance.off(LISTS_UPDATE_EVENT, this.onBreadcrumbsUpdate);
         OwnProfileStore.instance.off(UPDATE_EVENT, this.onBackgroundImageUpdate);
         SpaceStore.instance.off(UPDATE_SELECTED_SPACE, this.updateActiveSpace);
+        UIStore.instance.stopTrackingElementDimensions("LeftPanel");
     }
 
     private updateActiveSpace = (activeSpace: Room) => {
@@ -420,7 +426,7 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         );
 
         return (
-            <div className={containerClasses}>
+            <div className={containerClasses} ref={this.ref}>
                 {leftLeftPanel}
                 <aside className="mx_LeftPanel_roomListContainer">
                     {this.renderHeader()}

src/stores/UIStore.ts

@@ -24,12 +24,14 @@ export enum UI_EVENTS {
 
 export type ResizeObserverCallbackFunction = (entries: ResizeObserverEntry[]) => void;
 
-
 export default class UIStore extends EventEmitter {
     private static _instance: UIStore = null;
 
     private resizeObserver: ResizeObserver;
 
+    private uiElementDimensions = new Map<string, DOMRectReadOnly>();
+    private trackedUiElements = new Map<Element, string>();
+
     public windowWidth: number;
     public windowHeight: number;
 
@@ -60,14 +62,51 @@ export default class UIStore extends EventEmitter {
         }
     }
 
-    private resizeObserverCallback = (entries: ResizeObserverEntry[]) => {
+    public getElementDimensions(name: string): DOMRectReadOnly {
-        const { width, height } = entries
+        return this.uiElementDimensions.get(name);
-            .find(entry => entry.target === document.body)
+    }
-            .contentRect;
 
-        this.windowWidth = width;
+    public trackElementDimensions(name: string, element: Element): void {
-        this.windowHeight = height;
+        this.trackedUiElements.set(element, name);
+        this.resizeObserver.observe(element);
+    }
+
+    public stopTrackingElementDimensions(name: string): void {
+        let trackedElement: Element;
+        this.trackedUiElements.forEach((trackedElementName, element) => {
+            if (trackedElementName === name) {
+                trackedElement = element;
+            }
+        });
+        if (trackedElement) {
+            this.resizeObserver.unobserve(trackedElement);
+            this.uiElementDimensions.delete(name);
+            this.trackedUiElements.delete(trackedElement);
+        }
+    }
+
+    public isTrackingElementDimensions(name: string): boolean {
+        return this.uiElementDimensions.has(name);
+    }
+
+    private resizeObserverCallback = (entries: ResizeObserverEntry[]) => {
+        const windowEntry = entries.find(entry => entry.target === document.body);
+
+        if (windowEntry) {
+            this.windowWidth = windowEntry.contentRect.width;
+            this.windowHeight = windowEntry.contentRect.height;
+        }
+
+        entries.forEach(entry => {
+            const trackedElementName = this.trackedUiElements.get(entry.target);
+            if (trackedElementName) {
+                this.uiElementDimensions.set(trackedElementName, entry.contentRect);
+                this.emit(trackedElementName, UI_EVENTS.Resize, entry);
+            }
+        });
 
         this.emit(UI_EVENTS.Resize, entries);
     }
 }
+
+window.mxUIStore = UIStore.instance;