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.test.ts

@@ -0,0 +1,88 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import type { Room, MatrixEvent } from 'matrix-js-sdk';
+import { ReceiptType } from 'matrix-js-sdk/lib/@types/read_receipts';
+import { computeUpdatedPositions, getReceiptUserIds } from './useRoomReadPositions';
+
+// Fake, renderable (no reaction/edit relation) event.
+const fakeEvent = (id: string) =>
+  ({
+    getId: () => id,
+    getRelation: () => null,
+  }) as unknown as MatrixEvent;
+
+// Minimal fake room: a fixed live timeline plus a mutable per-user read-up-to map,
+// which the test mutates between calls to simulate new receipts arriving.
+const makeFakeRoom = (eventIds: string[]) => {
+  const readUpTo = new Map<string, string>();
+  const events = eventIds.map(fakeEvent);
+  const room = {
+    getLiveTimeline: () => ({ getEvents: () => events }),
+    getEventReadUpTo: (userId: string) => readUpTo.get(userId) ?? null,
+  } as unknown as Room;
+  return { room, readUpTo };
+};
+
+test('getReceiptUserIds collects every userId across events/types in the content', () => {
+  const content = {
+    $a: {
+      [ReceiptType.Read]: { '@alice:hs': { ts: 1 }, '@bob:hs': { ts: 2 } },
+    },
+    $b: {
+      [ReceiptType.ReadPrivate]: { '@carol:hs': { ts: 3 } },
+    },
+  };
+  const ids = getReceiptUserIds(content).sort();
+  assert.deepEqual(ids, ['@alice:hs', '@bob:hs', '@carol:hs']);
+});
+
+test('getReceiptUserIds returns empty for an empty receipt content', () => {
+  assert.deepEqual(getReceiptUserIds({}), []);
+});
+
+test('computeUpdatedPositions returns the SAME map reference when no named user changed position', () => {
+  const { room, readUpTo } = makeFakeRoom(['$a', '$b']);
+  readUpTo.set('@alice:hs', '$a');
+  const prev = computeUpdatedPositions(room, '@me:hs', new Map(), ['@alice:hs']);
+  assert.deepEqual(prev.get('$a'), ['@alice:hs']);
+
+  // Alice's read-up-to did not move; recompute should be a no-op reference-wise.
+  const next = computeUpdatedPositions(room, '@me:hs', prev, ['@alice:hs']);
+  assert.equal(next, prev);
+});
+
+test('computeUpdatedPositions moves only the named user, leaving other entries untouched by reference', () => {
+  const { room, readUpTo } = makeFakeRoom(['$a', '$b']);
+  readUpTo.set('@alice:hs', '$a');
+  readUpTo.set('@bob:hs', '$a');
+  const prev = computeUpdatedPositions(room, '@me:hs', new Map(), ['@alice:hs', '@bob:hs']);
+  const bArrayBefore = prev.get('$a');
+
+  // Only alice moves to $b.
+  readUpTo.set('@alice:hs', '$b');
+  const next = computeUpdatedPositions(room, '@me:hs', prev, ['@alice:hs']);
+
+  assert.notEqual(next, prev);
+  assert.deepEqual(next.get('$a'), ['@bob:hs']);
+  assert.deepEqual(next.get('$b'), ['@alice:hs']);
+  // Untouched arrays for other targets should keep their old reference where possible.
+  assert.notEqual(next.get('$a'), bArrayBefore); // this one WAS filtered, expected to change
+});
+
+test('computeUpdatedPositions ignores changedUserIds not present after filtering myUserId', () => {
+  const { room } = makeFakeRoom(['$a']);
+  const prev = new Map([['$a', ['@bob:hs']]]);
+  const next = computeUpdatedPositions(room, '@me:hs', prev, ['@me:hs']);
+  assert.equal(next, prev);
+});
+
+test('computeUpdatedPositions removes a user with no read-up-to event from the map', () => {
+  const { room, readUpTo } = makeFakeRoom(['$a']);
+  readUpTo.set('@alice:hs', '$a');
+  const prev = computeUpdatedPositions(room, '@me:hs', new Map(), ['@alice:hs']);
+  assert.deepEqual(prev.get('$a'), ['@alice:hs']);
+
+  readUpTo.delete('@alice:hs');
+  const next = computeUpdatedPositions(room, '@me:hs', prev, ['@alice:hs']);
+  assert.equal(next.has('$a'), false);
+});