refactor(sliding-sync): gate listTimelineLimit behind message preview settings

LIST_TIMELINE_LIMIT is now configurable via SlidingSyncConfig.listTimelineLimit
(default: 1). When dmMessagePreview or roomMessagePreview is enabled, the limit
is bumped to 5 so reactions/edits don't leave the preview empty. Users with both
preview settings disabled keep the lightweight limit of 1.

Author: Just-Insane

src/app/pages/client/ClientRoot.tsx

@@ -48,6 +48,7 @@ import { useSyncNicknames } from '$hooks/useNickname';
 import { useAppVisibility } from '$hooks/useAppVisibility';
 import { getHomePath } from '$pages/pathUtils';
 import { useClientConfig } from '$hooks/useClientConfig';
+import { getSettings } from '$state/settings';
 import { pushSessionToSW } from '../../../sw-session';
 import { SyncStatus } from './SyncStatus';
 import { SpecVersions } from './SpecVersions';
@@ -212,12 +213,18 @@ export function ClientRoot({ children }: ClientRootProps) {
 
   const [startState, startMatrix] = useAsyncCallback<void, Error, [MatrixClient]>(
     useCallback(
-      (m) =>
-        startClient(m, {
+      (m) => {
+        const s = getSettings();
+        const needsPreviewTimeline = s.dmMessagePreview || s.roomMessagePreview;
+        return startClient(m, {
           baseUrl: activeSession?.baseUrl,
-          slidingSync: clientConfig.slidingSync,
+          slidingSync: {
+            ...clientConfig.slidingSync,
+            listTimelineLimit: needsPreviewTimeline ? 5 : undefined,
+          },
           sessionSlidingSyncOptIn: activeSession?.slidingSyncOptIn,
-        }),
+        });
+      },
       [activeSession?.baseUrl, activeSession?.slidingSyncOptIn, clientConfig.slidingSync]
     )
   );

src/client/slidingSync.ts

@@ -36,10 +36,9 @@ export const LIST_ROOM_SEARCH = 'room_search';
 export const LIST_SPACE = 'space';
 // A small number of timeline events per list room. Unread counts come from
 // the server-side notification_count field, so a full history isn't needed.
-// We fetch a few events (rather than 1) so that reactions and edits — which
-// the SDK excludes from the main timeline when their parent event is absent —
-// don't leave the timeline empty and break message previews.
-const LIST_TIMELINE_LIMIT = 5;
+// When message previews are enabled, a higher limit (e.g. 5) avoids empty
+// timelines caused by reactions/edits whose parent event is absent.
+const DEFAULT_LIST_TIMELINE_LIMIT = 1;
 const DEFAULT_LIST_PAGE_SIZE = 250;
 const DEFAULT_POLL_TIMEOUT_MS = 20000;
 const DEFAULT_MAX_ROOMS = 5000;
@@ -53,7 +52,7 @@ const LIST_SORT_ORDER = ['by_recency', 'by_name'];
 // Encrypted rooms get [*,*] required_state; unencrypted rooms also request lazy members.
 const UNENCRYPTED_SUBSCRIPTION_KEY = 'unencrypted';
 // Timeline limit for the active-room subscription (full history load).
-// List entries always use LIST_TIMELINE_LIMIT=1 for lightweight previews.
+// List entries use a small timeline limit (default 1) for lightweight previews.
 const ACTIVE_ROOM_TIMELINE_LIMIT = 50;
 
 export type PartialSlidingSyncRequest = {
@@ -67,6 +66,7 @@ export type SlidingSyncConfig = {
   proxyBaseUrl?: string;
   bootstrapClassicOnColdCache?: boolean;
   listPageSize?: number;
+  listTimelineLimit?: number;
   timelineLimit?: number;
   pollTimeoutMs?: number;
   maxRooms?: number;
@@ -147,7 +147,7 @@ const buildUnencryptedSubscription = (timelineLimit: number): MSC3575RoomSubscri
   ],
 });
 
-const buildLists = (pageSize: number, includeInviteList: boolean): Map<string, MSC3575List> => {
+const buildLists = (pageSize: number, includeInviteList: boolean, listTimelineLimit: number): Map<string, MSC3575List> => {
   const lists = new Map<string, MSC3575List>();
   const listRequiredState = buildListRequiredState();
 
@@ -159,7 +159,7 @@ const buildLists = (pageSize: number, includeInviteList: boolean): Map<string, M
   lists.set(LIST_JOINED, {
     ranges: [[0, Math.max(0, initialRange - 1)]],
     sort: LIST_SORT_ORDER,
-    timeline_limit: LIST_TIMELINE_LIMIT,
+    timeline_limit: listTimelineLimit,
     required_state: listRequiredState,
     slow_get_all_rooms: true,
     filters: { is_invite: false },
@@ -169,7 +169,7 @@ const buildLists = (pageSize: number, includeInviteList: boolean): Map<string, M
     lists.set(LIST_INVITES, {
       ranges: [[0, Math.max(0, initialRange - 1)]],
       sort: LIST_SORT_ORDER,
-      timeline_limit: LIST_TIMELINE_LIMIT,
+      timeline_limit: listTimelineLimit,
       required_state: listRequiredState,
       slow_get_all_rooms: true,
       filters: { is_invite: true },
@@ -179,7 +179,7 @@ const buildLists = (pageSize: number, includeInviteList: boolean): Map<string, M
   lists.set(LIST_DMS, {
     ranges: [[0, Math.max(0, initialRange - 1)]],
     sort: LIST_SORT_ORDER,
-    timeline_limit: LIST_TIMELINE_LIMIT,
+    timeline_limit: listTimelineLimit,
     required_state: listRequiredState,
     slow_get_all_rooms: true,
     filters: { is_dm: true },
@@ -251,6 +251,8 @@ export class SlidingSyncManager {
 
   private readonly listPageSize: number;
 
+  private readonly listTimelineLimit: number;
+
   private readonly roomTimelineLimit: number;
 
   private readonly onConnectionChange: () => void;
@@ -303,12 +305,13 @@ export class SlidingSyncManager {
     this.maxRooms = clampPositive(config.maxRooms, DEFAULT_MAX_ROOMS);
     this.listPageSize = listPageSize;
     const includeInviteList = config.includeInviteList !== false;
+    this.listTimelineLimit = clampPositive(config.listTimelineLimit, DEFAULT_LIST_TIMELINE_LIMIT);
 
     const roomTimelineLimit = clampPositive(config.timelineLimit, ACTIVE_ROOM_TIMELINE_LIMIT);
     this.roomTimelineLimit = roomTimelineLimit;
 
     const defaultSubscription = buildEncryptedSubscription(roomTimelineLimit);
-    const lists = buildLists(listPageSize, includeInviteList);
+    const lists = buildLists(listPageSize, includeInviteList, this.listTimelineLimit);
     this.listKeys = Array.from(lists.keys());
     this.slidingSync = new SlidingSync(proxyBaseUrl, lists, defaultSubscription, mx, pollTimeoutMs);
 
@@ -720,7 +723,7 @@ export class SlidingSyncManager {
       list = {
         ranges: [[0, 20]],
         sort: LIST_SORT_ORDER,
-        timeline_limit: LIST_TIMELINE_LIMIT,
+        timeline_limit: this.listTimelineLimit,
         required_state: buildListRequiredState(),
         ...updateArgs,
       };