Use passive option for scroll handlers

res/css/views/rooms/_EventTile.scss

@@ -280,6 +280,7 @@ $left-gutter: 64px;
     height: $font-14px;
     width: $font-14px;
 
+    will-change: left, top;
     transition:
         left var(--transition-short) ease-out,
         top var(--transition-standard) ease-out;

src/CountlyAnalytics.ts

@@ -816,7 +816,9 @@ export default class CountlyAnalytics {
         window.addEventListener("mousemove", this.onUserActivity);
         window.addEventListener("click", this.onUserActivity);
         window.addEventListener("keydown", this.onUserActivity);
-        window.addEventListener("scroll", this.onUserActivity);
+        // Using the passive option to not block the main thread
+        // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners
+        window.addEventListener("scroll", this.onUserActivity, { passive: true });
 
         this.activityIntervalId = setInterval(() => {
             this.inactivityCounter++;

src/components/structures/AutoHideScrollbar.tsx

@@ -0,0 +1,65 @@
+/*
+Copyright 2018 New Vector Ltd
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+
+interface IProps {
+    className?: string;
+    onScroll?: () => void;
+    onWheel?: () => void;
+    style?: React.CSSProperties
+    tabIndex?: number,
+    wrappedRef?: (ref: HTMLDivElement) => void;
+}
+
+export default class AutoHideScrollbar extends React.Component<IProps> {
+    private containerRef: React.RefObject<HTMLDivElement> = React.createRef();
+
+    public componentDidMount() {
+        if (this.containerRef.current && this.props.onScroll) {
+            // Using the passive option to not block the main thread
+            // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners
+            this.containerRef.current.addEventListener("scroll", this.props.onScroll, { passive: true });
+        }
+
+        if (this.props.wrappedRef) {
+            this.props.wrappedRef(this.containerRef.current);
+        }
+    }
+
+    public componentWillUnmount() {
+        if (this.containerRef.current && this.props.onScroll) {
+            this.containerRef.current.removeEventListener("scroll", this.props.onScroll);
+        }
+    }
+
+    public getScrollTop(): number {
+        return this.containerRef.current.scrollTop;
+    }
+
+    public render() {
+        return (<div
+            ref={this.containerRef}
+            style={this.props.style}
+            className={["mx_AutoHideScrollbar", this.props.className].join(" ")}
+            onWheel={this.props.onWheel}
+            tabIndex={this.props.tabIndex}
+        >
+            { this.props.children }
+        </div>);
+    }
+}

src/components/structures/IndicatorScrollbar.js

@@ -59,7 +59,9 @@ export default class IndicatorScrollbar extends React.Component {
     _collectScroller(scroller) {
         if (scroller && !this._scrollElement) {
             this._scrollElement = scroller;
-            this._scrollElement.addEventListener("scroll", this.checkOverflow);
+            // Using the passive option to not block the main thread
+            // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners
+            this._scrollElement.addEventListener("scroll", this.checkOverflow, { passive: true });
             this.checkOverflow();
         }
     }

src/components/structures/LeftPanel.tsx

@@ -97,6 +97,9 @@ export default class LeftPanel extends React.Component<IProps, IState> {
     public componentDidMount() {
         UIStore.instance.trackElementDimensions("ListContainer", this.listContainerRef.current);
         UIStore.instance.on("ListContainer", this.refreshStickyHeaders);
+        // Using the passive option to not block the main thread
+        // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners
+        this.listContainerRef.current?.addEventListener("scroll", this.onScroll, { passive: true });
     }
 
     public componentWillUnmount() {
@@ -108,6 +111,7 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         SpaceStore.instance.off(UPDATE_SELECTED_SPACE, this.updateActiveSpace);
         UIStore.instance.stopTrackingElementDimensions("ListContainer");
         UIStore.instance.removeListener("ListContainer", this.refreshStickyHeaders);
+        this.listContainerRef.current?.removeEventListener("scroll", this.onScroll);
     }
 
     public componentDidUpdate(prevProps: IProps, prevState: IState): void {
@@ -295,7 +299,7 @@ export default class LeftPanel extends React.Component<IProps, IState> {
         }
     }
 
-    private onScroll = (ev: React.MouseEvent<HTMLDivElement>) => {
+    private onScroll = (ev: Event) => {
         const list = ev.target as HTMLDivElement;
         this.handleStickyHeaders(list);
     };
@@ -459,7 +463,6 @@ export default class LeftPanel extends React.Component<IProps, IState> {
                     <div className="mx_LeftPanel_roomListWrapper">
                         <div
                             className={roomListClasses}
-                            onScroll={this.onScroll}
                             ref={this.listContainerRef}
                             // Firefox sometimes makes this element focusable due to
                             // overflow:scroll;, so force it out of tab order.

src/components/views/dialogs/AddExistingToSpaceDialog.tsx

@@ -212,7 +212,7 @@ export const AddExistingToSpace: React.FC<IAddExistingToSpaceProps> = ({
             autoComplete={true}
             autoFocus={true}
         />
-        <AutoHideScrollbar className="mx_AddExistingToSpace_content" id="mx_AddExistingToSpace">
+        <AutoHideScrollbar className="mx_AddExistingToSpace_content">
             { rooms.length > 0 ? (
                 <div className="mx_AddExistingToSpace_section">
                     <h3>{ _t("Rooms") }</h3>

src/components/views/elements/Tooltip.tsx

@@ -70,7 +70,10 @@ export default class Tooltip extends React.Component<IProps> {
         this.tooltipContainer = document.createElement("div");
         this.tooltipContainer.className = "mx_Tooltip_wrapper";
         document.body.appendChild(this.tooltipContainer);
-        window.addEventListener('scroll', this.renderTooltip, true);
+        window.addEventListener('scroll', this.renderTooltip, {
+            passive: true,
+            capture: true,
+        });
 
         this.parent = ReactDOM.findDOMNode(this).parentNode as Element;
 
@@ -85,7 +88,9 @@ export default class Tooltip extends React.Component<IProps> {
     public componentWillUnmount() {
         ReactDOM.unmountComponentAtNode(this.tooltipContainer);
         document.body.removeChild(this.tooltipContainer);
-        window.removeEventListener('scroll', this.renderTooltip, true);
+        window.removeEventListener('scroll', this.renderTooltip, {
+            capture: true,
+        });
     }
 
     private updatePosition(style: CSSProperties) {

src/components/views/rooms/RoomSublist.tsx

@@ -105,6 +105,7 @@ interface IState {
 export default class RoomSublist extends React.Component<IProps, IState> {
     private headerButton = createRef<HTMLDivElement>();
     private sublistRef = createRef<HTMLDivElement>();
+    private tilesRef = createRef<HTMLDivElement>();
     private dispatcherRef: string;
     private layout: ListLayout;
     private heightAtStart: number;
@@ -246,11 +247,15 @@ export default class RoomSublist extends React.Component<IProps, IState> {
     public componentDidMount() {
         this.dispatcherRef = defaultDispatcher.register(this.onAction);
         RoomListStore.instance.on(LISTS_UPDATE_EVENT, this.onListsUpdated);
+        // Using the passive option to not block the main thread
+        // https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners
+        this.tilesRef.current?.addEventListener("scroll", this.onScrollPrevent, { passive: true });
     }
 
     public componentWillUnmount() {
         defaultDispatcher.unregister(this.dispatcherRef);
         RoomListStore.instance.off(LISTS_UPDATE_EVENT, this.onListsUpdated);
+        this.tilesRef.current?.removeEventListener("scroll", this.onScrollPrevent);
     }
 
     private onListsUpdated = () => {
@@ -755,7 +760,7 @@ export default class RoomSublist extends React.Component<IProps, IState> {
         );
     }
 
-    private onScrollPrevent(e: React.UIEvent<HTMLDivElement>) {
+    private onScrollPrevent(e: Event) {
         // the RoomTile calls scrollIntoView and the browser may scroll a div we do not wish to be scrollable
         // this fixes https://github.com/vector-im/element-web/issues/14413
         (e.target as HTMLDivElement).scrollTop = 0;
@@ -884,7 +889,7 @@ export default class RoomSublist extends React.Component<IProps, IState> {
                         className="mx_RoomSublist_resizeBox"
                         enable={handles}
                     >
-                        <div className="mx_RoomSublist_tiles" onScroll={this.onScrollPrevent}>
+                        <div className="mx_RoomSublist_tiles" ref={this.tilesRef}>
                             {visibleTiles}
                         </div>
                         {showNButton}