fix(desktop): preserve buffered channel unread

Co-authored-by: Morgan Martin <morgan@block.xyz>
Signed-off-by: Morgan Martin <morgan@block.xyz>

desktop/src/features/messages/ui/MessageTimeline.tsx

@@ -23,6 +23,7 @@ import { TimelineSkeleton, useTimelineSkeletonRows } from "./TimelineSkeleton";
 import { TimelineMessageList } from "./TimelineMessageList";
 import type { TimelineVirtualizerApi } from "./TimelineMessageList";
 import { useAnchoredScroll } from "./useAnchoredScroll";
+import { resolveTimelineReadPresence } from "./anchoredScrollPolicy";
 import { useLoadOlderOnScroll } from "./useLoadOlderOnScroll";
 import { useBufferedTimelineMessages } from "./useBufferedTimelineMessages";
 import {
@@ -372,8 +373,17 @@ const MessageTimelineBase = React.forwardRef<
   });
 
   React.useEffect(() => {
-    onAtBottomChange?.(isAtBottom);
-  }, [isAtBottom, onAtBottomChange]);
+    // Read presence follows the semantic tail, not the physical virtualizer
+    // floor. While live arrivals are buffered, freezing the rendered tail can
+    // make the shortened DOM report physically at-bottom even though the
+    // reader is still scrolled away from the new messages.
+    onAtBottomChange?.(
+      resolveTimelineReadPresence({
+        isPhysicallyAtBottom: isAtBottom,
+        isSemanticallyAtBottom,
+      }),
+    );
+  }, [isAtBottom, isSemanticallyAtBottom, onAtBottomChange]);
   const hasConfirmedVirtualizerBottomRef = React.useRef(false);
   const bottomConfirmationChannelRef = React.useRef(channelId);
   if (bottomConfirmationChannelRef.current !== channelId) {

desktop/src/features/messages/ui/anchoredScrollPolicy.ts

@@ -42,6 +42,18 @@ export function settleProgrammaticBottomPin(
   );
 }
 
+export function resolveTimelineReadPresence({
+  isPhysicallyAtBottom: _isPhysicallyAtBottom,
+  isSemanticallyAtBottom,
+}: {
+  isPhysicallyAtBottom: boolean;
+  isSemanticallyAtBottom: boolean;
+}): boolean {
+  // A buffered timeline can shorten the rendered model enough that the
+  // virtualizer reports its physical floor while newer messages remain hidden.
+  return isSemanticallyAtBottom;
+}
+
 export function shouldSettleForSplitPanel({
   isAtBottom,
   splitPanelOpen,

desktop/src/features/messages/ui/useAnchoredScroll.test.mjs

@@ -3,6 +3,7 @@ import test from "node:test";
 
 import {
   getPinnedCenterDrift,
+  resolveTimelineReadPresence,
   settleProgrammaticBottomPin,
   shouldIgnorePinnedCenterScroll,
   shouldSettleForSplitPanel,
@@ -23,6 +24,23 @@ function fakeContainer({ clientHeight, scrollHeight, scrollTop }) {
   };
 }
 
+test("buffered arrivals do not become read when the shortened timeline reports its physical floor", () => {
+  assert.equal(
+    resolveTimelineReadPresence({
+      isPhysicallyAtBottom: true,
+      isSemanticallyAtBottom: false,
+    }),
+    false,
+  );
+  assert.equal(
+    resolveTimelineReadPresence({
+      isPhysicallyAtBottom: true,
+      isSemanticallyAtBottom: true,
+    }),
+    true,
+  );
+});
+
 test("split panel settles only an already-bottomed timeline", () => {
   assert.equal(
     shouldSettleForSplitPanel({ isAtBottom: true, splitPanelOpen: true }),