Merge pull request #6658 from matrix-org/gsouquet/threaded-messaging-2349

.stylelintrc.js

@@ -17,6 +17,7 @@ module.exports = {
         "selector-list-comma-newline-after": null,
         "at-rule-no-unknown": null,
         "no-descending-specificity": null,
+        "no-empty-first-line": true,
         "scss/at-rule-no-unknown": [true, {
             // https://github.com/vector-im/element-web/issues/10544
             "ignoreAtRules": ["define-mixin"],

res/css/views/messages/_MessageActionBar.scss

@@ -92,6 +92,10 @@ limitations under the License.
     mask-image: url('$(res)/img/element-icons/room/message-bar/reply.svg');
 }
 
+.mx_MessageActionBar_threadButton::after {
+    mask-image: url('$(res)/img/element-icons/message/thread.svg');
+}
+
 .mx_MessageActionBar_editButton::after {
     mask-image: url('$(res)/img/element-icons/room/message-bar/edit.svg');
 }

res/css/views/right_panel/_RoomSummaryCard.scss

@@ -232,6 +232,10 @@ limitations under the License.
     mask-image: url('$(res)/img/element-icons/room/files.svg');
 }
 
+.mx_RoomSummaryCard_icon_threads::before {
+    mask-image: url('$(res)/img/element-icons/message/thread.svg');
+}
+
 .mx_RoomSummaryCard_icon_share::before {
     mask-image: url('$(res)/img/element-icons/room/share.svg');
 }

res/css/views/rooms/_EventTile.scss

@@ -643,6 +643,7 @@ $hover-select-border: 4px;
 
     // Remove some of the default tile padding so that the error is centered
     margin-right: 0;
+
     .mx_EventTile_line {
         padding-left: 0;
         margin-right: 0;
@@ -674,3 +675,62 @@ $hover-select-border: 4px;
         margin-right: 0;
     }
 }
+
+.mx_ThreadInfo:hover {
+    cursor: pointer;
+}
+
+.mx_ThreadView {
+    display: flex;
+    flex-direction: column;
+
+    .mx_ScrollPanel {
+        margin-top: 20px;
+
+        .mx_RoomView_MessageList {
+            padding: 0;
+        }
+    }
+
+    .mx_EventTile_senderDetails {
+        display: flex;
+        align-items: center;
+        gap: 6px;
+        margin-bottom: 6px;
+
+        a {
+            flex: 1;
+            min-width: none;
+            max-width: 100%;
+            display: flex;
+            align-items: center;
+
+            .mx_SenderProfile {
+                flex: 1;
+            }
+        }
+    }
+
+    .mx_ThreadView_List {
+        flex: 1;
+        overflow: scroll;
+    }
+
+    .mx_EventTile_roomName {
+        display: none;
+    }
+
+    .mx_EventTile_line {
+        padding-left: 0 !important;
+        order: 10 !important;
+    }
+
+    .mx_EventTile {
+        width: 100%;
+        display: flex;
+        flex-direction: column;
+        margin-top: 0;
+        padding-bottom: 5px;
+        margin-bottom: 5px;
+    }
+}

res/css/views/rooms/_MessageComposer.scss

@@ -340,3 +340,19 @@ limitations under the License.
         height: 50px;
     }
 }
+
+/**
+ * Unstable compact mode
+ */
+
+.mx_MessageComposer.mx_MessageComposer--compact {
+    margin-right: 0;
+
+    .mx_MessageComposer_wrapper {
+        padding: 0;
+    }
+
+    .mx_MessageComposer_button:last-child {
+        margin-right: 0;
+    }
+}

res/img/element-icons/message/thread.svg

@@ -0,0 +1,4 @@
+<svg width="18" height="18" viewBox="0 0 18 18" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M3 8V8C1.89543 8 1 7.10457 1 6V3C1 1.89543 1.89543 1 3 1H15C16.1046 1 17 1.89484 17 2.9994C17 3.88147 17 4.95392 17 6.00008C17 7.10465 16.1046 8 15 8H10.5" stroke="#737D8C" stroke-width="1.5" stroke-linecap="round"/>
+<path fill-rule="evenodd" clip-rule="evenodd" d="M13.2011 16.7176C12.9087 17.011 12.9088 17.4866 13.2012 17.78C13.4936 18.0734 13.9677 18.0733 14.2601 17.78C14.9484 17.0894 15.6519 16.3829 16.1834 15.8491L16.8282 15.2014L17.0099 15.0188L17.0579 14.9706L17.0702 14.9582L17.0733 14.955L17.0741 14.9542L17.0743 14.954L17.0743 14.954L16.5444 14.4233L17.0744 14.954C17.3663 14.6606 17.3661 14.1855 17.0741 13.8922L14.2539 11.061C13.9616 10.7675 13.4875 10.7674 13.195 11.0606C12.9024 11.3539 12.9023 11.8295 13.1946 12.123L14.7442 13.6787L10.1137 13.6787C8.69795 13.6787 7.49996 12.4759 7.49996 10.9288L7.49996 7.00002C7.49996 6.58581 7.16417 6.25002 6.74996 6.25002C6.33574 6.25002 5.99996 6.58581 5.99996 7.00002L5.99996 10.9288C5.99996 13.2476 7.81395 15.1787 10.1137 15.1787H14.7341C14.2713 15.6436 13.7316 16.1854 13.2011 16.7176Z" fill="#737D8C"/>
+</svg>

src/MatrixClientPeg.ts

@@ -213,6 +213,7 @@ class MatrixClientPegClass implements IMatrixClientPeg {
         opts.pendingEventOrdering = PendingEventOrdering.Detached;
         opts.lazyLoadMembers = true;
         opts.clientWellKnownPollPeriod = 2 * 60 * 60; // 2 hours
+        opts.experimentalThreadSupport = SettingsStore.getValue("feature_thread");
 
         // Connect the matrix client to the dispatcher and setting handlers
         MatrixActionCreators.start(this.matrixClient);

src/components/structures/MessagePanel.tsx

@@ -173,6 +173,8 @@ interface IProps {
     onUnfillRequest?(backwards: boolean, scrollToken: string): void;
 
     getRelationsForEvent?(eventId: string, relationType: string, eventType: string): Relations;
+
+    hideThreadedMessages?: boolean;
 }
 
 interface IState {
@@ -265,6 +267,9 @@ export default class MessagePanel extends React.Component<IProps, IState> {
     componentDidMount() {
         this.calculateRoomMembersCount();
         this.props.room?.on("RoomState.members", this.calculateRoomMembersCount);
+        if (SettingsStore.getValue("feature_thread")) {
+            this.props.room?.getThreads().forEach(thread => thread.fetchReplyChain());
+        }
         this.isMounted = true;
     }
 
@@ -443,6 +448,12 @@ export default class MessagePanel extends React.Component<IProps, IState> {
         // Always show highlighted event
         if (this.props.highlightedEventId === mxEv.getId()) return true;
 
+        if (mxEv.replyEventId
+                && this.props.hideThreadedMessages
+                && SettingsStore.getValue("feature_thread")) {
+            return false;
+        }
+
         return !shouldHideEvent(mxEv, this.context);
     }
 

src/components/structures/RightPanel.tsx

@@ -45,17 +45,21 @@ import GroupRoomInfo from "../views/groups/GroupRoomInfo";
 import UserInfo from "../views/right_panel/UserInfo";
 import ThirdPartyMemberInfo from "../views/rooms/ThirdPartyMemberInfo";
 import FilePanel from "./FilePanel";
+import ThreadView from "./ThreadView";
+import ThreadPanel from "./ThreadPanel";
 import NotificationPanel from "./NotificationPanel";
 import ResizeNotifier from "../../utils/ResizeNotifier";
 import PinnedMessagesCard from "../views/right_panel/PinnedMessagesCard";
 import { throttle } from 'lodash';
 import SpaceStore from "../../stores/SpaceStore";
+import { RoomPermalinkCreator } from '../../utils/permalinks/Permalinks';
 
 interface IProps {
     room?: Room; // if showing panels for a given room, this is set
     groupId?: string; // if showing panels for a given group, this is set
     user?: User; // used if we know the user ahead of opening the panel
     resizeNotifier: ResizeNotifier;
+    permalinkCreator?: RoomPermalinkCreator;
 }
 
 interface IState {
@@ -309,6 +313,22 @@ export default class RightPanel extends React.Component<IProps, IState> {
                 panel = <FilePanel roomId={roomId} resizeNotifier={this.props.resizeNotifier} onClose={this.onClose} />;
                 break;
 
+            case RightPanelPhases.ThreadView:
+                panel = <ThreadView
+                    room={this.props.room}
+                    resizeNotifier={this.props.resizeNotifier}
+                    onClose={this.onClose}
+                    mxEvent={this.state.event}
+                    permalinkCreator={this.props.permalinkCreator} />;
+                break;
+
+            case RightPanelPhases.ThreadPanel:
+                panel = <ThreadPanel
+                    roomId={roomId}
+                    resizeNotifier={this.props.resizeNotifier}
+                    onClose={this.onClose} />;
+                break;
+
             case RightPanelPhases.RoomSummary:
                 panel = <RoomSummaryCard room={this.props.room} onClose={this.onClose} />;
                 break;

src/components/structures/RoomView.tsx

@@ -2052,7 +2052,10 @@ export default class RoomView extends React.Component<IProps, IState> {
 
         const showRightPanel = this.state.room && this.state.showRightPanel;
         const rightPanel = showRightPanel
-            ? <RightPanel room={this.state.room} resizeNotifier={this.props.resizeNotifier} />
+            ? <RightPanel
+                room={this.state.room}
+                resizeNotifier={this.props.resizeNotifier}
+                permalinkCreator={this.getPermalinkCreatorForRoom(this.state.room)} />
             : null;
 
         const timelineClasses = classNames("mx_RoomView_timeline", {

src/components/structures/ThreadPanel.tsx

@@ -0,0 +1,93 @@
+/*
+Copyright 2021 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';
+import { MatrixEvent, Room } from 'matrix-js-sdk/src';
+import { Thread } from 'matrix-js-sdk/src/models/thread';
+
+import BaseCard from "../views/right_panel/BaseCard";
+import { RightPanelPhases } from "../../stores/RightPanelStorePhases";
+import { replaceableComponent } from "../../utils/replaceableComponent";
+import { MatrixClientPeg } from '../../MatrixClientPeg';
+
+import ResizeNotifier from '../../utils/ResizeNotifier';
+import EventTile from '../views/rooms/EventTile';
+
+interface IProps {
+    roomId: string;
+    onClose: () => void;
+    resizeNotifier: ResizeNotifier;
+}
+
+interface IState {
+    threads?: Thread[];
+}
+
+@replaceableComponent("structures.ThreadView")
+export default class ThreadPanel extends React.Component<IProps, IState> {
+    private room: Room;
+
+    constructor(props: IProps) {
+        super(props);
+        this.room = MatrixClientPeg.get().getRoom(this.props.roomId);
+    }
+
+    public componentDidMount(): void {
+        this.room.on("Thread.update", this.onThreadEventReceived);
+        this.room.on("Thread.ready", this.onThreadEventReceived);
+    }
+
+    public componentWillUnmount(): void {
+        this.room.removeListener("Thread.update", this.onThreadEventReceived);
+        this.room.removeListener("Thread.ready", this.onThreadEventReceived);
+    }
+
+    private onThreadEventReceived = () => this.updateThreads();
+
+    private updateThreads = (callback?: () => void): void => {
+        this.setState({
+            threads: this.room.getThreads(),
+        }, callback);
+    };
+
+    private renderEventTile(event: MatrixEvent): JSX.Element {
+        return <EventTile
+            key={event.getId()}
+            mxEvent={event}
+            enableFlair={false}
+            showReadReceipts={false}
+            as="div"
+        />;
+    }
+
+    public render(): JSX.Element {
+        return (
+            <BaseCard
+                className="mx_ThreadPanel"
+                onClose={this.props.onClose}
+                previousPhase={RightPanelPhases.RoomSummary}
+            >
+                {
+                    this.state?.threads.map((thread: Thread) => {
+                        if (thread.ready) {
+                            return this.renderEventTile(thread.rootEvent);
+                        }
+                    })
+                }
+            </BaseCard>
+        );
+    }
+}

src/components/structures/ThreadView.tsx

@@ -0,0 +1,147 @@
+/*
+Copyright 2021 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';
+import { MatrixEvent, Room } from 'matrix-js-sdk/src';
+import { Thread } from 'matrix-js-sdk/src/models/thread';
+
+import BaseCard from "../views/right_panel/BaseCard";
+import { RightPanelPhases } from "../../stores/RightPanelStorePhases";
+import { replaceableComponent } from "../../utils/replaceableComponent";
+
+import ResizeNotifier from '../../utils/ResizeNotifier';
+import { TileShape } from '../views/rooms/EventTile';
+import MessageComposer from '../views/rooms/MessageComposer';
+import { RoomPermalinkCreator } from '../../utils/permalinks/Permalinks';
+import { Layout } from '../../settings/Layout';
+import TimelinePanel from './TimelinePanel';
+import dis from "../../dispatcher/dispatcher";
+import { ActionPayload } from '../../dispatcher/payloads';
+import { SetRightPanelPhasePayload } from '../../dispatcher/payloads/SetRightPanelPhasePayload';
+import { Action } from '../../dispatcher/actions';
+
+interface IProps {
+    room: Room;
+    onClose: () => void;
+    resizeNotifier: ResizeNotifier;
+    mxEvent: MatrixEvent;
+    permalinkCreator?: RoomPermalinkCreator;
+}
+
+interface IState {
+    replyToEvent?: MatrixEvent;
+    thread?: Thread;
+}
+
+@replaceableComponent("structures.ThreadView")
+export default class ThreadView extends React.Component<IProps, IState> {
+    private dispatcherRef: string;
+
+    constructor(props: IProps) {
+        super(props);
+        this.state = {};
+    }
+
+    public componentDidMount(): void {
+        this.setupThread(this.props.mxEvent);
+        this.dispatcherRef = dis.register(this.onAction);
+    }
+
+    public componentWillUnmount(): void {
+        this.teardownThread();
+        dis.unregister(this.dispatcherRef);
+    }
+
+    public componentDidUpdate(prevProps) {
+        if (prevProps.mxEvent !== this.props.mxEvent) {
+            this.teardownThread();
+            this.setupThread(this.props.mxEvent);
+        }
+
+        if (prevProps.room !== this.props.room) {
+            dis.dispatch<SetRightPanelPhasePayload>({
+                action: Action.SetRightPanelPhase,
+                phase: RightPanelPhases.RoomSummary,
+            });
+        }
+    }
+
+    private onAction = (payload: ActionPayload): void => {
+        if (payload.phase == RightPanelPhases.ThreadView && payload.event) {
+            if (payload.event !== this.props.mxEvent) {
+                this.teardownThread();
+                this.setupThread(payload.event);
+            }
+        }
+    };
+
+    private setupThread = (mxEv: MatrixEvent) => {
+        const thread = mxEv.getThread();
+        if (thread) {
+            thread.on("Thread.update", this.updateThread);
+            thread.once("Thread.ready", this.updateThread);
+            this.updateThread(thread);
+        }
+    };
+
+    private teardownThread = () => {
+        if (this.state.thread) {
+            this.state.thread.removeListener("Thread.update", this.updateThread);
+            this.state.thread.removeListener("Thread.ready", this.updateThread);
+        }
+    };
+
+    private updateThread = (thread?: Thread) => {
+        if (thread) {
+            this.setState({ thread });
+        } else {
+            this.forceUpdate();
+        }
+    };
+
+    public render(): JSX.Element {
+        return (
+            <BaseCard
+                className="mx_ThreadView"
+                onClose={this.props.onClose}
+                previousPhase={RightPanelPhases.RoomSummary}
+                withoutScrollContainer={true}
+            >
+                { this.state.thread && (
+                    <TimelinePanel
+                        manageReadReceipts={false}
+                        manageReadMarkers={false}
+                        timelineSet={this.state?.thread?.timelineSet}
+                        showUrlPreview={false}
+                        tileShape={TileShape.Notif}
+                        empty={<div>empty</div>}
+                        alwaysShowTimestamps={true}
+                        layout={Layout.Group}
+                        hideThreadedMessages={false}
+                    />
+                ) }
+                <MessageComposer
+                    room={this.props.room}
+                    resizeNotifier={this.props.resizeNotifier}
+                    replyToEvent={this.state?.thread?.replyToEvent}
+                    showReplyPreview={false}
+                    permalinkCreator={this.props.permalinkCreator}
+                    compact={true}
+                />
+            </BaseCard>
+        );
+    }
+}

src/components/structures/TimelinePanel.tsx

@@ -126,6 +126,8 @@ interface IProps {
 
     // callback which is called when we wish to paginate the timeline window.
     onPaginationRequest?(timelineWindow: TimelineWindow, direction: string, size: number): Promise<boolean>;
+
+    hideThreadedMessages?: boolean;
 }
 
 interface IState {
@@ -214,6 +216,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
         timelineCap: Number.MAX_VALUE,
         className: 'mx_RoomView_messagePanel',
         sendReadReceiptOnLoad: true,
+        hideThreadedMessages: true,
     };
 
     private lastRRSentEventId: string = undefined;
@@ -1511,6 +1514,7 @@ class TimelinePanel extends React.Component<IProps, IState> {
                 showReactions={this.props.showReactions}
                 layout={this.props.layout}
                 enableFlair={SettingsStore.getValue(UIFeature.Flair)}
+                hideThreadedMessages={this.props.hideThreadedMessages}
             />
         );
     }

src/components/views/messages/MessageActionBar.js

@@ -23,6 +23,8 @@ import { EventStatus } from 'matrix-js-sdk/src/models/event';
 import { _t } from '../../../languageHandler';
 import * as sdk from '../../../index';
 import dis from '../../../dispatcher/dispatcher';
+import { Action } from '../../../dispatcher/actions';
+import { RightPanelPhases } from '../../../stores/RightPanelStorePhases';
 import { aboveLeftOf, ContextMenu, ContextMenuTooltipButton, useContextMenu } from '../../structures/ContextMenu';
 import { isContentActionable, canEditContent } from '../../../utils/EventUtils';
 import RoomContext from "../../../contexts/RoomContext";
@@ -34,6 +36,7 @@ import Resend from "../../../Resend";
 import { MatrixClientPeg } from "../../../MatrixClientPeg";
 import { MediaEventHelper } from "../../../utils/MediaEventHelper";
 import DownloadActionButton from "./DownloadActionButton";
+import SettingsStore from '../../../settings/SettingsStore';
 
 const OptionsButton = ({ mxEvent, getTile, getReplyThread, permalinkCreator, onFocusChange }) => {
     const [menuDisplayed, button, openMenu, closeMenu] = useContextMenu();
@@ -170,6 +173,17 @@ export default class MessageActionBar extends React.PureComponent {
         });
     };
 
+    onThreadClick = () => {
+        dis.dispatch({
+            action: Action.SetRightPanelPhase,
+            phase: RightPanelPhases.ThreadView,
+            allowClose: false,
+            refireParams: {
+                event: this.props.mxEvent,
+            },
+        });
+    }
+
     onEditClick = (ev) => {
         dis.dispatch({
             action: 'edit_event',
@@ -254,12 +268,22 @@ export default class MessageActionBar extends React.PureComponent {
                 // The only catch is we do the reply button first so that we can make sure the react
                 // button is the very first button without having to do length checks for `splice()`.
                 if (this.context.canReply) {
-                    toolbarOpts.splice(0, 0, <RovingAccessibleTooltipButton
-                        className="mx_MessageActionBar_maskButton mx_MessageActionBar_replyButton"
-                        title={_t("Reply")}
-                        onClick={this.onReplyClick}
-                        key="reply"
-                    />);
+                    toolbarOpts.splice(0, 0, <>
+                        <RovingAccessibleTooltipButton
+                            className="mx_MessageActionBar_maskButton mx_MessageActionBar_replyButton"
+                            title={_t("Reply")}
+                            onClick={this.onReplyClick}
+                            key="reply"
+                        />
+                        { SettingsStore.getValue("feature_thread") && (
+                            <RovingAccessibleTooltipButton
+                                className="mx_MessageActionBar_maskButton mx_MessageActionBar_threadButton"
+                                title={_t("Thread")}
+                                onClick={this.onThreadClick}
+                                key="thread"
+                            />
+                        ) }
+                    </>);
                 }
                 if (this.context.canReact) {
                     toolbarOpts.splice(0, 0, <ReactButton

src/components/views/right_panel/RoomSummaryCard.tsx

@@ -220,6 +220,13 @@ const onRoomFilesClick = () => {
     });
 };
 
+const onRoomThreadsClick = () => {
+    defaultDispatcher.dispatch<SetRightPanelPhasePayload>({
+        action: Action.SetRightPanelPhase,
+        phase: RightPanelPhases.ThreadPanel,
+    });
+};
+
 const onRoomSettingsClick = () => {
     defaultDispatcher.dispatch({ action: "open_room_settings" });
 };
@@ -273,6 +280,11 @@ const RoomSummaryCard: React.FC<IProps> = ({ room, onClose }) => {
             <Button className="mx_RoomSummaryCard_icon_files" onClick={onRoomFilesClick}>
                 { _t("Show files") }
             </Button>
+            { SettingsStore.getValue("feature_thread") && (
+                <Button className="mx_RoomSummaryCard_icon_threads" onClick={onRoomThreadsClick}>
+                    { _t("Show threads") }
+                </Button>
+            ) }
             <Button className="mx_RoomSummaryCard_icon_share" onClick={onShareRoomClick}>
                 { _t("Share room") }
             </Button>

src/components/views/rooms/EventTile.tsx

@@ -21,6 +21,7 @@ import { EventType } from "matrix-js-sdk/src/@types/event";
 import { EventStatus, MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { Relations } from "matrix-js-sdk/src/models/relations";
 import { RoomMember } from "matrix-js-sdk/src/models/room-member";
+import { Thread } from 'matrix-js-sdk/src/models/thread';
 
 import ReplyThread from "../elements/ReplyThread";
 import { _t } from '../../../languageHandler';
@@ -55,6 +56,8 @@ import ReadReceiptMarker from "./ReadReceiptMarker";
 import MessageActionBar from "../messages/MessageActionBar";
 import ReactionsRow from '../messages/ReactionsRow';
 import { getEventDisplayInfo } from '../../../utils/EventUtils';
+import { RightPanelPhases } from "../../../stores/RightPanelStorePhases";
+import SettingsStore from "../../../settings/SettingsStore";
 
 const eventTileTypes = {
     [EventType.RoomMessage]: 'messages.MessageEvent',
@@ -299,6 +302,9 @@ interface IProps {
 
     // whether or not to display the sender
     hideSender?: boolean;
+
+    // whether or not to display thread info
+    showThreadInfo?: boolean;
 }
 
 interface IState {
@@ -315,6 +321,8 @@ interface IState {
     reactions: Relations;
 
     hover: boolean;
+
+    thread?: Thread;
 }
 
 @replaceableComponent("views.rooms.EventTile")
@@ -351,6 +359,8 @@ export default class EventTile extends React.Component<IProps, IState> {
             reactions: this.getReactions(),
 
             hover: false,
+
+            thread: this.props.mxEvent?.getThread(),
         };
 
         // don't do RR animations until we are mounted
@@ -451,8 +461,20 @@ export default class EventTile extends React.Component<IProps, IState> {
             client.on("Room.receipt", this.onRoomReceipt);
             this.isListeningForReceipts = true;
         }
+
+        if (SettingsStore.getValue("feature_thread")) {
+            this.props.mxEvent.once("Thread.ready", this.updateThread);
+            this.props.mxEvent.on("Thread.update", this.updateThread);
+        }
     }
 
+    private updateThread = (thread) => {
+        this.setState({
+            thread,
+        });
+        this.forceUpdate();
+    };
+
     // TODO: [REACT-WARNING] Replace with appropriate lifecycle event
     // eslint-disable-next-line
     UNSAFE_componentWillReceiveProps(nextProps) {
@@ -463,7 +485,7 @@ export default class EventTile extends React.Component<IProps, IState> {
         }
     }
 
-    shouldComponentUpdate(nextProps, nextState) {
+    shouldComponentUpdate(nextProps, nextState, nextContext) {
         if (objectHasDiff(this.state, nextState)) {
             return true;
         }
@@ -491,6 +513,43 @@ export default class EventTile extends React.Component<IProps, IState> {
         }
     }
 
+    private renderThreadInfo(): React.ReactNode {
+        if (!SettingsStore.getValue("feature_thread")) {
+            return null;
+        }
+
+        const thread = this.state.thread;
+        const room = MatrixClientPeg.get().getRoom(this.props.mxEvent.getRoomId());
+        if (!thread || this.props.showThreadInfo === false) {
+            return null;
+        }
+
+        const avatars = Array.from(thread.participants).map((mxId: string) => {
+            const member = room.getMember(mxId);
+            return <MemberAvatar key={member.userId} member={member} width={14} height={14} />;
+        });
+
+        return (
+            <div
+                className="mx_ThreadInfo"
+                onClick={() => {
+                    dis.dispatch({
+                        action: Action.SetRightPanelPhase,
+                        phase: RightPanelPhases.ThreadView,
+                        refireParams: {
+                            event: this.props.mxEvent,
+                        },
+                    });
+                }}
+            >
+                <span className="mx_EventListSummary_avatars">
+                    { avatars }
+                </span>
+                { thread.length - 1 } { thread.length === 2 ? 'reply' : 'replies' }
+            </div>
+        );
+    }
+
     private onRoomReceipt = (ev, room) => {
         // ignore events for other rooms
         const tileRoom = MatrixClientPeg.get().getRoom(this.props.mxEvent.getRoomId());
@@ -1180,6 +1239,7 @@ export default class EventTile extends React.Component<IProps, IState> {
                             { keyRequestInfo }
                             { actionBar }
                             { this.props.layout === Layout.IRC && (reactionsRow) }
+                            { this.renderThreadInfo() }
                         </div>
                         { this.props.layout !== Layout.IRC && (reactionsRow) }
                         { msgOption }

src/components/views/rooms/MessageComposer.tsx

@@ -183,7 +183,9 @@ interface IProps {
     resizeNotifier: ResizeNotifier;
     permalinkCreator: RoomPermalinkCreator;
     replyToEvent?: MatrixEvent;
+    showReplyPreview?: boolean;
     e2eStatus?: E2EStatus;
+    compact?: boolean;
 }
 
 interface IState {
@@ -201,6 +203,11 @@ export default class MessageComposer extends React.Component<IProps, IState> {
     private messageComposerInput: SendMessageComposer;
     private voiceRecordingButton: VoiceRecordComposerTile;
 
+    static defaultProps = {
+        showReplyPreview: true,
+        compact: false,
+    };
+
     constructor(props) {
         super(props);
         VoiceRecordingStore.instance.on(UPDATE_EVENT, this.onVoiceStoreUpdate);
@@ -362,7 +369,7 @@ export default class MessageComposer extends React.Component<IProps, IState> {
 
     render() {
         const controls = [
-            this.state.me ? <ComposerAvatar key="controls_avatar" me={this.state.me} /> : null,
+            this.state.me && !this.props.compact ? <ComposerAvatar key="controls_avatar" me={this.state.me} /> : null,
             this.props.e2eStatus ?
                 <E2EIcon key="e2eIcon" status={this.props.e2eStatus} className="mx_MessageComposer_e2eIcon" /> :
                 null,
@@ -450,11 +457,19 @@ export default class MessageComposer extends React.Component<IProps, IState> {
             />;
         }
 
+        const classes = classNames({
+            "mx_MessageComposer": true,
+            "mx_GroupLayout": true,
+            "mx_MessageComposer--compact": this.props.compact,
+        });
+
         return (
-            <div className="mx_MessageComposer mx_GroupLayout">
+            <div className={classes}>
                 { recordingTooltip }
                 <div className="mx_MessageComposer_wrapper">
-                    <ReplyPreview permalinkCreator={this.props.permalinkCreator} />
+                    { this.props.showReplyPreview && (
+                        <ReplyPreview permalinkCreator={this.props.permalinkCreator} />
+                    ) }
                     <div className="mx_MessageComposer_row">
                         { controls }
                     </div>

src/i18n/strings/en_EN.json

@@ -806,6 +806,7 @@
     "Render LaTeX maths in messages": "Render LaTeX maths in messages",
     "Communities v2 prototypes. Requires compatible homeserver. Highly experimental - use with caution.": "Communities v2 prototypes. Requires compatible homeserver. Highly experimental - use with caution.",
     "Message Pinning": "Message Pinning",
+    "Threaded messaging": "Threaded messaging",
     "Custom user status messages": "Custom user status messages",
     "Group & filter rooms by custom tags (refresh to apply changes)": "Group & filter rooms by custom tags (refresh to apply changes)",
     "Render simple counters in room header": "Render simple counters in room header",
@@ -1803,6 +1804,7 @@
     "%(count)s people|other": "%(count)s people",
     "%(count)s people|one": "%(count)s person",
     "Show files": "Show files",
+    "Show threads": "Show threads",
     "Share room": "Share room",
     "Room settings": "Room settings",
     "Trusted": "Trusted",
@@ -1922,6 +1924,7 @@
     "React": "React",
     "Edit": "Edit",
     "Reply": "Reply",
+    "Thread": "Thread",
     "Message Actions": "Message Actions",
     "Download %(text)s": "Download %(text)s",
     "Error decrypting attachment": "Error decrypting attachment",

src/settings/Settings.tsx

@@ -211,6 +211,15 @@ export const SETTINGS: {[setting: string]: ISetting} = {
         supportedLevels: LEVELS_FEATURE,
         default: false,
     },
+    "feature_thread": {
+        isFeature: true,
+        // Requires a reload as we change an option flag on the `js-sdk`
+        // And the entire sync history needs to be parsed again
+        controller: new ReloadOnChangeController(),
+        displayName: _td("Threaded messaging"),
+        supportedLevels: LEVELS_FEATURE,
+        default: false,
+    },
     "feature_custom_status": {
         isFeature: true,
         displayName: _td("Custom user status messages"),

src/stores/RightPanelStorePhases.ts

@@ -37,6 +37,10 @@ export enum RightPanelPhases {
     SpaceMemberList = "SpaceMemberList",
     SpaceMemberInfo = "SpaceMemberInfo",
     Space3pidMemberInfo = "Space3pidMemberInfo",
+
+    // Thread stuff
+    ThreadView = "ThreadView",
+    ThreadPanel = "ThreadPanel",
 }
 
 // These are the phases that are safe to persist (the ones that don't require additional