Remove forgotten rooms from the room list once forgotten

Fixes https://github.com/vector-im/element-web/issues/15559

This isn't exactly perfect as an implementation: if the user refreshes immediately after forgetting then there is a good chance the room re-appears because of the sync accumulator. At the very least this change makes it so in *most* cases the room goes away, which is probably good enough until https://github.com/vector-im/element-web/issues/14038 can be implemented properly.

src/components/structures/MatrixChat.tsx

@@ -85,6 +85,8 @@ import { showToast as showMobileGuideToast } from '../../toasts/MobileGuideToast
 import SpaceStore from "../../stores/SpaceStore";
 import SpaceRoomDirectory from "./SpaceRoomDirectory";
 import {replaceableComponent} from "../../utils/replaceableComponent";
+import RoomListStore from "../../stores/room-list/RoomListStore";
+import {RoomUpdateCause} from "../../stores/room-list/models";
 
 /** constants for MatrixChat.state.view */
 export enum Views {
@@ -1140,11 +1142,17 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
     }
 
     private forgetRoom(roomId: string) {
+        const room = MatrixClientPeg.get().getRoom(roomId);
         MatrixClientPeg.get().forget(roomId).then(() => {
             // Switch to home page if we're currently viewing the forgotten room
             if (this.state.currentRoomId === roomId) {
                 dis.dispatch({ action: "view_home_page" });
             }
+
+            // We have to manually update the room list because the forgotten room will not
+            // be notified to us, therefore the room list will have no other way of knowing
+            // the room is forgotten.
+            RoomListStore.instance.manualRoomUpdate(room, RoomUpdateCause.RoomRemoved);
         }).catch((err) => {
             const errCode = err.errcode || _td("unknown error code");
             Modal.createTrackedDialog("Failed to forget room", '', ErrorDialog, {

src/stores/room-list/RoomListStore.ts

@@ -655,6 +655,18 @@ export class RoomListStoreClass extends AsyncStoreWithClient<IState> {
         if (!algorithmTags) return [DefaultTagID.Untagged];
         return algorithmTags;
     }
+
+    /**
+     * Manually update a room with a given cause. This should only be used if the
+     * room list store would otherwise be incapable of doing the update itself. Note
+     * that this may race with the room list's regular operation.
+     * @param {Room} room The room to update.
+     * @param {RoomUpdateCause} cause The cause to update for.
+     */
+    public async manualRoomUpdate(room: Room, cause: RoomUpdateCause) {
+        await this.handleRoomUpdate(room, cause);
+        this.updateFn.trigger();
+    }
 }
 
 export default class RoomListStore {