fix(timeline): thread panel shows read receipts; receipts recompute incrementally

- ReadPositionsContext is provided once at Room level so the thread
  panel (a sibling of RoomView) gets real positions instead of the empty
  default; own thread messages no longer sit on "Sent" forever (#38).
- Receipt events only recompute the users they name, merged into the
  previous map with reference equality preserved for untouched rows, so
  a receipt no longer re-renders every message (#40). Unit-tested.

Fixes #38
Fixes #40

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/hooks/useRoomReadPositions.ts

@@ -1,4 +1,5 @@
 import { Room, RoomEvent, RoomMember, RoomMemberEvent, MatrixEvent } from 'matrix-js-sdk';
+import { ReceiptContent } from 'matrix-js-sdk/lib/@types/read_receipts';
 import { useEffect, useState } from 'react';
 import { useMatrixClient } from './useMatrixClient';
 import { reactionOrEditEvent } from '../utils/room';
@@ -19,6 +20,18 @@ function nearestRenderableId(
   return null;
 }
 
+// A `m.receipt` event's content names every user whose receipt moved, keyed by
+// eventId -> receiptType -> userId. Exported for testing.
+export function getReceiptUserIds(content: ReceiptContent): string[] {
+  const userIds = new Set<string>();
+  Object.values(content).forEach((receiptsByType) => {
+    Object.values(receiptsByType).forEach((receiptsByUser) => {
+      Object.keys(receiptsByUser).forEach((userId) => userIds.add(userId));
+    });
+  });
+  return Array.from(userIds);
+}
+
 function computePositions(room: Room, myUserId: string): Map<string, string[]> {
   const map = new Map<string, string[]>();
   const liveEvents = room.getLiveTimeline().getEvents();
@@ -37,6 +50,64 @@ function computePositions(room: Room, myUserId: string): Map<string, string[]> {
   return map;
 }
 
+// Recompute positions for only the given users, reusing the previous Map/arrays for
+// everyone else. Returns the SAME `prevMap` reference when nothing actually changed,
+// so unaffected `Message`s (the overwhelming majority on every receipt) skip re-render
+// (Gitea #40).
+export function computeUpdatedPositions(
+  room: Room,
+  myUserId: string,
+  prevMap: Map<string, string[]>,
+  changedUserIds: Iterable<string>,
+): Map<string, string[]> {
+  const usersToUpdate = new Set(changedUserIds);
+  usersToUpdate.delete(myUserId);
+  if (usersToUpdate.size === 0) return prevMap;
+
+  const liveEvents = room.getLiveTimeline().getEvents();
+  const eventIndex = new Map<string, number>(liveEvents.map((e, i) => [e.getId() ?? '', i]));
+
+  // Each changed user's current target, so a receipt that didn't actually move
+  // them past the previously-computed nearest renderable event is a true no-op.
+  const oldTargetByUser = new Map<string, string>();
+  prevMap.forEach((users, targetId) => {
+    users.forEach((u) => {
+      if (usersToUpdate.has(u)) oldTargetByUser.set(u, targetId);
+    });
+  });
+
+  // Copy lazily: stay on `prevMap` (and its untouched per-event arrays) unless a
+  // user actually moved, so unrelated Messages keep the same array/Map reference.
+  let nextMap = prevMap;
+  const ensureCopy = (): Map<string, string[]> => {
+    if (nextMap === prevMap) nextMap = new Map(prevMap);
+    return nextMap;
+  };
+
+  usersToUpdate.forEach((userId) => {
+    const evtId = room.getEventReadUpTo(userId);
+    const newTargetId = evtId ? nearestRenderableId(liveEvents, eventIndex, evtId) : null;
+    const oldTargetId = oldTargetByUser.get(userId) ?? null;
+    if (newTargetId === oldTargetId) return;
+
+    const map = ensureCopy();
+    if (oldTargetId) {
+      const users = map.get(oldTargetId);
+      if (users) {
+        const filtered = users.filter((u) => u !== userId);
+        if (filtered.length === 0) map.delete(oldTargetId);
+        else map.set(oldTargetId, filtered);
+      }
+    }
+    if (newTargetId) {
+      const users = map.get(newTargetId);
+      map.set(newTargetId, users ? [...users, userId] : [userId]);
+    }
+  });
+
+  return nextMap;
+}
+
 export function useRoomReadPositions(room: Room): Map<string, string[]> {
   const mx = useMatrixClient();
   const myUserId = mx.getUserId() ?? '';
@@ -45,11 +116,19 @@ export function useRoomReadPositions(room: Room): Map<string, string[]> {
   useEffect(() => {
     setPositions(computePositions(room, myUserId));
     let debounceTimer: ReturnType<typeof setTimeout> | null = null;
-    const onReceipt = (): void => {
+    // Accumulated across every receipt event that lands during the debounce window so
+    // a burst of receipts still only touches the users actually named in them.
+    const pendingUserIds = new Set<string>();
+    const onReceipt = (event: MatrixEvent): void => {
+      getReceiptUserIds(event.getContent<ReceiptContent>()).forEach((userId) =>
+        pendingUserIds.add(userId),
+      );
       if (debounceTimer !== null) clearTimeout(debounceTimer);
       debounceTimer = setTimeout(() => {
-        setPositions(computePositions(room, myUserId));
+        const userIds = Array.from(pendingUserIds);
+        pendingUserIds.clear();
         debounceTimer = null;
+        setPositions((prev) => computeUpdatedPositions(room, myUserId, prev, userIds));
       }, 150);
     };
     // RoomMemberEvent.Membership is emitted on the RoomMember (and re-emitted on the