fix(desktop): reconcile thread arrivals at bottom (#3585)

## Summary
- reconcile stale native-scroll anchors when a reply arrives at the
physical floor
- clear the thread new-message affordance instead of incrementing it
from stale cached state
- preserve the existing mid-history path and add direct lifecycle
regression coverage

## Why
PR #3411 fixed geometry-driven reconciliation, but the reply-arrival
branch still trusted a cached `message` anchor without checking the
rendered position. Native anchoring could return a short thread to the
floor without another scroll/resize callback, then the next reply
incremented the pill anyway.

## Verification
- Desktop checks passed
- Desktop typecheck passed
- focused lifecycle test passed (6/6)
- push hook full Desktop unit suite passed (3,770/3,770)
- `git diff --check` passed

Signed-off-by: Wes <wesbillman@users.noreply.github.com>
Co-authored-by: Carl <c7ebe626f000404285d3686e1dc74cc07cc60a9754a150041ba132e14bd3e2ec@buzz.block.builderlab.xyz>

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

@@ -247,13 +247,20 @@ function Harness({ channelId, onTargetSettled, refs }) {
   return null;
 }
 
-function BottomStateHarness({ messages, onState, refs }) {
+function BottomStateHarness({
+  messages,
+  onState,
+  refs,
+  targetMessageId = null,
+}) {
   const anchored = useAnchoredScroll({
     channelId: "conversation",
     contentRef: refs.content,
     isLoading: false,
     messages,
+    pinTargetCentered: targetMessageId !== null,
     scrollContainerRef: refs.container,
+    targetMessageId,
   });
   onState(anchored);
   return null;
@@ -329,6 +336,90 @@ test("channel change attaches pinned-center observers after refs mount", async (
   });
 });
 
+test("arrival at the physical floor does not preserve a stale unread state", async () => {
+  const refs = {
+    container: { current: null },
+    content: { current: null },
+  };
+  const root = createRoot(document.createElement("div"));
+  const nodes = makePinnedCenterNodes();
+  refs.container.current = nodes.container;
+  refs.content.current = nodes.content;
+  let state = null;
+  const render = (messages) =>
+    root.render(
+      React.createElement(BottomStateHarness, {
+        messages,
+        onState: (nextState) => {
+          state = nextState;
+        },
+        refs,
+      }),
+    );
+
+  await act(async () => render([{ id: "first" }]));
+  await act(async () => new Promise((resolve) => setTimeout(resolve, 0)));
+  nodes.container.scrollTop = 100;
+  await act(async () => state.onScroll());
+  nodes.container.scrollTop = 100;
+  await act(async () => state.onScroll());
+  assert.equal(state.isAtBottom, false);
+
+  // Native anchoring can return the viewport to the floor without a scroll or
+  // resize callback, leaving only the hook's cached message anchor stale.
+  nodes.container.scrollTop =
+    nodes.container.scrollHeight - nodes.container.clientHeight;
+  await act(async () => render([{ id: "first" }, { id: "second" }]));
+
+  assert.equal(state.isAtBottom, true);
+  assert.equal(state.newMessageCount, 0);
+  await act(async () => root.unmount());
+});
+
+test("arrival does not steal an active layout target during floor-like reflow", async () => {
+  const refs = {
+    container: { current: null },
+    content: { current: null },
+  };
+  const root = createRoot(document.createElement("div"));
+  const nodes = makePinnedCenterNodes();
+  refs.container.current = nodes.container;
+  refs.content.current = nodes.content;
+  let state = null;
+  const render = (messages, targetMessageId = null) =>
+    root.render(
+      React.createElement(BottomStateHarness, {
+        messages,
+        onState: (nextState) => {
+          state = nextState;
+        },
+        refs,
+        targetMessageId,
+      }),
+    );
+
+  await act(async () => render([{ id: "selected" }]));
+  await act(async () => new Promise((resolve) => setTimeout(resolve, 0)));
+  nodes.container.scrollTop = 100;
+  await act(async () => state.onScroll());
+  nodes.container.scrollTop = 100;
+  await act(async () => state.onScroll());
+  assert.equal(state.isAtBottom, false);
+
+  // A focus/split presentation switch can commit fresh replies while the old
+  // container geometry momentarily reads as the physical floor. The explicit
+  // layout target must win so the reading row is restored after reflow.
+  nodes.container.scrollTop =
+    nodes.container.scrollHeight - nodes.container.clientHeight;
+  await act(async () =>
+    render([{ id: "selected" }, { id: "second" }], "selected"),
+  );
+
+  assert.equal(state.isAtBottom, false);
+  assert.equal(state.newMessageCount, 1);
+  await act(async () => root.unmount());
+});
+
 test("container resize clears a stale new-message state at the physical floor", async () => {
   const refs = {
     container: { current: null },

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

@@ -714,6 +714,22 @@ export function useAnchoredScroll({
         container.scrollTo({ top: container.scrollHeight, behavior: "auto" });
       }
       if (newLatestArrived) setNewMessageCount(0);
+    } else if (
+      messagesArrived > 0 &&
+      !targetMessageId &&
+      !virtualizerOwnsPrependAnchoring &&
+      isAtBottomNow(container)
+    ) {
+      // A native scroll/layout callback may not have reconciled a stale
+      // message anchor before this append commits. If the rendered result is
+      // still physically at the floor (common in short threads), do not turn
+      // that stale anchor into a visible unread affordance. Active navigation
+      // targets own the viewport and must be preserved across presentation
+      // reflow even when the old geometry momentarily reads as the floor.
+      anchorRef.current = { kind: "at-bottom" };
+      container.scrollTo({ top: container.scrollHeight, behavior: "auto" });
+      setIsAtBottom(true);
+      setNewMessageCount(0);
     } else if (messagesArrived > 0 && !virtualizerOwnsPrependAnchoring) {
       // Anchored mid-history. An older-history prepend grows the content above
       // the reading row; the browser's native scroll anchoring does NOT correct