Turn previews on for DMs and ensure they get rendered

src/components/views/rooms/RoomTile2.tsx

@@ -54,6 +54,7 @@ import defaultDispatcher from "../../../dispatcher/dispatcher";
 import {ActionPayload} from "../../../dispatcher/payloads";
 import { RoomNotificationStateStore } from "../../../stores/notifications/RoomNotificationStateStore";
 import { NotificationState } from "../../../stores/notifications/NotificationState";
+import { UPDATE_EVENT } from "../../../stores/AsyncStore";
 
 // TODO: Rename on launch: https://github.com/vector-im/riot-web/issues/14367
 
@@ -128,6 +129,7 @@ export default class RoomTile2 extends React.Component<IProps, IState> {
         };
 
         ActiveRoomObserver.addListener(this.props.room.roomId, this.onActiveRoomUpdate);
+        MessagePreviewStore.instance.on(UPDATE_EVENT, this.onPreviewUpdated);
         this.dispatcherRef = defaultDispatcher.register(this.onAction);
     }
 
@@ -150,9 +152,14 @@ export default class RoomTile2 extends React.Component<IProps, IState> {
         if (this.props.room) {
             ActiveRoomObserver.removeListener(this.props.room.roomId, this.onActiveRoomUpdate);
         }
+        MessagePreviewStore.instance.off(UPDATE_EVENT, this.onPreviewUpdated);
         defaultDispatcher.unregister(this.dispatcherRef);
     }
 
+    private onPreviewUpdated = () => {
+        this.forceUpdate(); // we don't track the preview in state, so just re-render
+    };
+
     private onAction = (payload: ActionPayload) => {
         if (payload.action === "view_room" && payload.room_id === this.props.room.roomId && payload.show_room_tile) {
             setImmediate(() => {

src/stores/room-list/ListLayout.ts

@@ -14,7 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { TagID } from "./models";
+import { DefaultTagID, TagID } from "./models";
+import { isNullOrUndefined } from "matrix-js-sdk/src/utils";
 
 const TILE_HEIGHT_PX = 44;
 
@@ -26,7 +27,7 @@ interface ISerializedListLayout {
 
 export class ListLayout {
     private _n = 0;
-    private _previews = false;
+    private _previews: boolean | null = null;
     private _collapsed = false;
 
     constructor(public readonly tagId: TagID) {
@@ -50,7 +51,12 @@ export class ListLayout {
     }
 
     public get showPreviews(): boolean {
-        return this._previews;
+        if (!isNullOrUndefined(this._previews)) {
+            return this._previews;
+        }
+
+        // Turn it on for DMs by default, but not for other rooms
+        return this.tagId === DefaultTagID.DM;
     }
 
     public set showPreviews(v: boolean) {