fix(notifications/threads): Wave-1 audit fixes (🔴 + web 🟠)

- T1 (🔴): markThreadAsRead no longer receipts the thread ROOT (a 2nd instance
  of the read-marker-corruption regression — opening a thread whose root is old
  re-lit the whole room). Extracted to a pure threadReceipt.ts + 5 regression
  tests.
- N1 (🔴): favicon/tab-title unread count now sums only leaf rooms (was double-
  counting every ancestor-space aggregate in roomToUnread).
- N2 (🔴): notifications/sounds dedupe on the event id, not the unread count —
  fixes "read a DM, next message never notifies again".
- T4 (🟠): the thread notification path no longer re-gates on the room count, so
  an explicit per-thread "All replies" override in a Mentions-only room fires.
- N3 (🟠): getUnreadInfos skips phantom {0,0} entries (muted-thread-only rooms no
  longer light the nav row / pollute unread filters).
- N4 (🟠): the Receipt handler recomputes unread instead of blanket-DELETE, so a
  threaded receipt can't wipe a room's valid main-timeline badge.
- T2 (🟠): thread "Jump to Latest" re-anchors the virtual window (was landing on
  a stale mid/old event).

Gates: tsc/eslint/prettier clean, build OK, 678 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/thread/ThreadTimeline.tsx

@@ -460,12 +460,17 @@ export function ThreadTimeline({ room, thread, editor }: ThreadTimelineProps) {
   }, [scrollToBottomCount]);
 
   const handleJumpToBottom = useCallback(() => {
+    // Re-anchor the virtual window at the thread tail first. While scrolled up,
+    // live replies deliberately don't extend the window, so without this the chip
+    // would scroll to the bottom of the STALE window (a mid/old event) instead of
+    // the newest reply. Mirrors the main timeline's handleJumpToLatest.
+    setTimeline(getInitialThreadTimeline(thread, getLinkedTimelines(thread.liveTimeline)));
     scrollToBottomRef.current.count += 1;
     scrollToBottomRef.current.smooth = true;
     // Flip atBottom so the layout effect re-runs (count re-read) and live
     // events resume sticking to the bottom.
     setAtBottom(true);
-  }, []);
+  }, [thread]);
 
   // Scroll in-place editor into view.
   useEffect(() => {

src/app/features/room/thread/threadReceipt.test.ts

@@ -0,0 +1,55 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { ReceiptType } from 'matrix-js-sdk';
+import { markThreadAsRead } from './threadReceipt';
+
+// The regression this guards: sending a receipt for the thread ROOT (when
+// replies aren't loaded, lastReply() is null / equals the root) becomes a MAIN
+// receipt at an old event and drags the room's read marker backwards. It must
+// only ever receipt a genuine loaded reply.
+
+const evt = (id: string, sending = false) => ({ getId: () => id, isSending: () => sending }) as any;
+
+const setup = (lastReply: any) => {
+  const calls: Array<{ eventId: string; type: ReceiptType }> = [];
+  const thread = { id: '$root', lastReply: () => lastReply } as any;
+  const mx = {
+    sendReadReceipt: async (e: any, type: ReceiptType) => {
+      calls.push({ eventId: e.getId(), type });
+      return {};
+    },
+  } as any;
+  return { mx, thread, calls };
+};
+
+test('REGRESSION: no loaded reply (lastReply null) → NO receipt (never the root)', async () => {
+  const { mx, thread, calls } = setup(null);
+  await markThreadAsRead(mx, thread, false);
+  assert.equal(calls.length, 0);
+});
+
+test('REGRESSION: lastReply IS the root → NO receipt', async () => {
+  const { mx, thread, calls } = setup(evt('$root'));
+  await markThreadAsRead(mx, thread, false);
+  assert.equal(calls.length, 0);
+});
+
+test('genuine loaded reply → threaded receipt at that reply', async () => {
+  const { mx, thread, calls } = setup(evt('$reply'));
+  await markThreadAsRead(mx, thread, false);
+  assert.equal(calls.length, 1);
+  assert.equal(calls[0].eventId, '$reply');
+  assert.equal(calls[0].type, ReceiptType.Read);
+});
+
+test('sending reply is skipped', async () => {
+  const { mx, thread, calls } = setup(evt('$reply', true));
+  await markThreadAsRead(mx, thread, false);
+  assert.equal(calls.length, 0);
+});
+
+test('private flag uses ReadPrivate', async () => {
+  const { mx, thread, calls } = setup(evt('$reply'));
+  await markThreadAsRead(mx, thread, true);
+  assert.equal(calls[0].type, ReceiptType.ReadPrivate);
+});

src/app/features/room/thread/threadReceipt.ts

@@ -0,0 +1,28 @@
+import { MatrixClient, ReceiptType, Thread } from 'matrix-js-sdk';
+
+/**
+ * Send a threaded read receipt for a thread, clearing its per-thread unread
+ * count.
+ *
+ * CRITICAL: never receipt the thread ROOT. A thread's liveTimeline is
+ * `[root, reply1, …]`, so the latest event IS the root when replies aren't
+ * loaded yet (common — the thread panel fires this on mount before replies
+ * fetch). The root is "in the main timeline", so a receipt for it is written by
+ * the SDK with `thread_id:"main"` at the old root, dragging the room's MAIN read
+ * marker backwards (`getEventReadUpTo` → an old/unloaded event) and re-lighting
+ * the whole room. We only receipt a genuine loaded reply (`thread.lastReply()`);
+ * if none is loaded we bail (the per-thread count clears when the reply loads
+ * and this runs again). Mirrors the root guard in `utils/notifications.ts`.
+ *
+ * Pure (no React/CSS) so it can be unit-tested — see `threadReceipt.test.ts`.
+ */
+export const markThreadAsRead = async (
+  mx: MatrixClient,
+  thread: Thread,
+  privateReceipt: boolean,
+): Promise<void> => {
+  const lastReply = thread.lastReply();
+  if (!lastReply || lastReply.isSending() || lastReply.getId() === thread.id) return;
+
+  await mx.sendReadReceipt(lastReply, privateReceipt ? ReceiptType.ReadPrivate : ReceiptType.Read);
+};

src/app/features/room/thread/useThread.ts

@@ -4,7 +4,6 @@ import {
   EventTimeline,
   MatrixClient,
   MatrixEvent,
-  ReceiptType,
   Room,
   RoomEvent,
   RoomEventHandlerMap,
@@ -146,32 +145,6 @@ export const useThreadPendingEvents = (
   return pending;
 };
 
-/**
- * Send a threaded read receipt up to the latest confirmed event in the thread.
- *
- * The receipt is threaded by default (scoped to this thread), which clears the
- * per-thread unread count. Mirrors the latest-valid-event scan in
- * `utils/notifications.ts`.
- */
-export const markThreadAsRead = async (
-  mx: MatrixClient,
-  thread: Thread,
-  privateReceipt: boolean,
-): Promise<void> => {
-  const events = thread.liveTimeline.getEvents();
-
-  let latestEvent: MatrixEvent | undefined;
-  for (let i = events.length - 1; i >= 0; i -= 1) {
-    const evt = events[i];
-    if (evt && !evt.isSending()) {
-      latestEvent = evt;
-      break;
-    }
-  }
-  if (!latestEvent) return;
-
-  await mx.sendReadReceipt(
-    latestEvent,
-    privateReceipt ? ReceiptType.ReadPrivate : ReceiptType.Read,
-  );
-};
+// markThreadAsRead moved to ./threadReceipt (pure + unit-tested); re-exported
+// here for existing import sites.
+export { markThreadAsRead } from './threadReceipt';