Add jump to bottom button for right panel timeline (#7746)

res/css/views/right_panel/_TimelineCard.scss

@@ -112,3 +112,8 @@ limitations under the License.
         flex-basis: 48px; // 12 (padding on message list) + 36 (padding on event lines)
     }
 }
+
+.mx_TimelineCard_timeline {
+    overflow: hidden;
+    position: relative; // offset parent for jump to bottom button
+}

src/components/views/right_panel/TimelineCard.tsx

@@ -16,7 +16,9 @@ limitations under the License.
 
 import React from 'react';
 import { EventSubscription } from "fbemitter";
-import { EventTimelineSet, IEventRelation, MatrixEvent, Room } from 'matrix-js-sdk/src';
+import { IEventRelation, MatrixEvent } from 'matrix-js-sdk/src/models/event';
+import { EventTimelineSet } from 'matrix-js-sdk/src/models/event-timeline-set';
+import { NotificationCountType, Room } from 'matrix-js-sdk/src/models/room';
 import { Thread } from 'matrix-js-sdk/src/models/thread';
 import classNames from 'classnames';
 
@@ -38,6 +40,7 @@ import RoomViewStore from '../../../stores/RoomViewStore';
 import ContentMessages from '../../../ContentMessages';
 import UploadBar from '../../structures/UploadBar';
 import SettingsStore from '../../../settings/SettingsStore';
+import JumpToBottomButton from '../rooms/JumpToBottomButton';
 
 interface IProps {
     room: Room;
@@ -58,6 +61,7 @@ interface IState {
     initialEventId?: string;
     isInitialEventHighlighted?: boolean;
     layout: Layout;
+    atEndOfLiveTimeline: boolean;
 
     // settings:
     showReadReceipts?: boolean;
@@ -78,6 +82,7 @@ export default class TimelineCard extends React.Component<IProps, IState> {
         this.state = {
             showReadReceipts: SettingsStore.getValue("showReadReceipts", props.room.roomId),
             layout: SettingsStore.getValue("layout"),
+            atEndOfLiveTimeline: true,
         };
         this.readReceiptsSettingWatcher = null;
     }
@@ -137,7 +142,7 @@ export default class TimelineCard extends React.Component<IProps, IState> {
         }
     };
 
-    private onScroll = (): void => {
+    private onUserScroll = (): void => {
         if (this.state.initialEventId && this.state.isInitialEventHighlighted) {
             dis.dispatch({
                 action: Action.ViewRoom,
@@ -149,6 +154,36 @@ export default class TimelineCard extends React.Component<IProps, IState> {
         }
     };
 
+    private onScroll = (): void => {
+        const timelinePanel = this.timelinePanelRef.current;
+        if (!timelinePanel) return;
+        if (timelinePanel.isAtEndOfLiveTimeline()) {
+            this.setState({
+                atEndOfLiveTimeline: true,
+            });
+        } else {
+            this.setState({
+                atEndOfLiveTimeline: false,
+            });
+        }
+    };
+
+    private jumpToLiveTimeline = () => {
+        if (this.state.initialEventId && this.state.isInitialEventHighlighted) {
+            // If we were viewing a highlighted event, firing view_room without
+            // an event will take care of both clearing the URL fragment and
+            // jumping to the bottom
+            dis.dispatch({
+                action: Action.ViewRoom,
+                room_id: this.props.room.roomId,
+            });
+        } else {
+            // Otherwise we have to jump manually
+            this.timelinePanelRef.current?.jumpToLiveTimeline();
+            dis.fire(Action.FocusSendMessageComposer);
+        }
+    };
+
     private renderTimelineCardHeader = (): JSX.Element => {
         return <div className="mx_TimelineCard__header">
             <span>{ _t("Chat") }</span>
@@ -165,6 +200,14 @@ export default class TimelineCard extends React.Component<IProps, IState> {
             "mx_GroupLayout": this.state.layout === Layout.Group,
         });
 
+        let jumpToBottom;
+        if (!this.state.atEndOfLiveTimeline) {
+            jumpToBottom = (<JumpToBottomButton
+                highlight={this.props.room.getUnreadNotificationCount(NotificationCountType.Highlight) > 0}
+                onScrollToBottomClick={this.jumpToLiveTimeline}
+            />);
+        }
+
         return (
             <RoomContext.Provider value={{
                 ...this.context,
@@ -177,6 +220,8 @@ export default class TimelineCard extends React.Component<IProps, IState> {
                     withoutScrollContainer={true}
                     header={this.renderTimelineCardHeader()}
                 >
+                    <div className="mx_TimelineCard_timeline">
+                        { jumpToBottom }
                         <TimelinePanel
                             ref={this.timelinePanelRef}
                             showReadReceipts={this.state.showReadReceipts}
@@ -197,8 +242,10 @@ export default class TimelineCard extends React.Component<IProps, IState> {
                             eventId={this.state.initialEventId}
                             resizeNotifier={this.props.resizeNotifier}
                             highlightedEventId={highlightedEventId}
-                        onUserScroll={this.onScroll}
+                            onScroll={this.onScroll}
+                            onUserScroll={this.onUserScroll}
                         />
+                    </div>
 
                     { ContentMessages.sharedInstance().getCurrentUploads(this.props.composerRelation).length > 0 && (
                         <UploadBar room={this.props.room} relation={this.props.composerRelation} />

src/components/views/rooms/JumpToBottomButton.tsx

@@ -21,7 +21,7 @@ import { _t } from '../../../languageHandler';
 import AccessibleButton from '../elements/AccessibleButton';
 
 interface IProps {
-    numUnreadMessages: number;
+    numUnreadMessages?: number;
     highlight: boolean;
     onScrollToBottomClick: (e: React.MouseEvent) => void;
 }