fix(#4778): scroll-rebuild guard must not reclassify a pinned follower as unpinned

Codex regression gate caught: the guard predicate keyed on a raw messagesEl.scrollTop>0,
so a pinned reader transiently at bottomDistance>250 mid-render (assistant body grows
before the anchor scene re-renders) got flipped to _messageUserUnpinned=true/_scrollPinned=false
and lost auto-follow. Require an explicit non-follow signal (_messageUserUnpinned || _scrollPinned===false)
instead. Updated the existing guard test to a genuine unpinned reader + added a pinned-large-growth
regression. Co-authored-by: franksong2702

static/ui.js

@@ -9595,7 +9595,12 @@ function _prepareLiveAnchorScrollRebuildGuard(scrollSnapshot){
   const messagesEl=$('messages');
   if(!messagesEl||!scrollSnapshot) return {readerAwayFromBottom:false,release:null};
   const beforeBottomDistance=Math.max(0,messagesEl.scrollHeight-messagesEl.scrollTop-messagesEl.clientHeight);
-  const readerAwayFromBottom=beforeBottomDistance>250&&(_messageUserUnpinned||messagesEl.scrollTop>0);
+  // Only treat the reader as away if they were ALREADY in a non-follow state.
+  // A pinned follower can transiently have bottomDistance>250 mid-render (the
+  // assistant body grows before the anchor scene re-renders), so keying on a
+  // raw scrollTop>0 here would mis-classify a pinned reader as unpinned and kill
+  // auto-follow. Require an explicit unpin/non-pinned signal instead.
+  const readerAwayFromBottom=beforeBottomDistance>250&&(_messageUserUnpinned||_scrollPinned===false);
   if(!readerAwayFromBottom) return {readerAwayFromBottom:false,release:null};
   scrollSnapshot.pinned=false;
   scrollSnapshot.userUnpinned=true;

tests/test_issue3479_ios_stream_scroll_jump.py

@@ -85,7 +85,7 @@ def test_live_anchor_worklog_rebuild_guards_height_for_unpinned_reader():
     compact = _compact(guard)
 
     assert "constbeforeBottomDistance=Math.max(0,messagesEl.scrollHeight-messagesEl.scrollTop-messagesEl.clientHeight);" in compact
-    assert "beforeBottomDistance>250&&(_messageUserUnpinned||messagesEl.scrollTop>0)" in compact
+    assert "beforeBottomDistance>250&&(_messageUserUnpinned||_scrollPinned===false)" in compact
     assert "scrollSnapshot.pinned=false;" in compact
     assert "scrollSnapshot.userUnpinned=true;" in compact
     assert "scrollSnapshot.bottom=beforeBottomDistance;" in compact

tests/test_issue4295_midstream_scroll_anchor.py

@@ -112,8 +112,8 @@ def test_live_anchor_rebuild_guard_holds_height_and_marks_reader_unpinned():
 
     script = f"""
 const assert = require('assert');
-let _messageUserUnpinned = false;
-let _scrollPinned = true;
+let _messageUserUnpinned = true;
+let _scrollPinned = false;
 let _nearBottomCount = 2;
 const messages = {{
   scrollTop: 2000,
@@ -165,6 +165,50 @@ assert.strictEqual(Object.prototype.hasOwnProperty.call(inner.dataset, 'liveAnch
     subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)
 
 
+def test_live_anchor_rebuild_guard_keeps_pinned_follower_pinned_on_large_growth():
+    """A PINNED follower must NOT be reclassified as unpinned when a large live
+    render transiently pushes bottomDistance>250. Regression guard: the predicate
+    must require an explicit non-follow signal (_messageUserUnpinned / _scrollPinned
+    === false), not a raw scrollTop>0, or pinned live streams stop auto-following."""
+
+    script = f"""
+const assert = require('assert');
+let _messageUserUnpinned = false;
+let _scrollPinned = true;
+let _nearBottomCount = 2;
+const messages = {{
+  scrollTop: 4400,
+  scrollHeight: 5500,
+  clientHeight: 600,
+}};
+const inner = {{ style: {{ minHeight: '' }}, dataset: {{}} }};
+function $(id) {{
+  if (id === 'messages') return messages;
+  if (id === 'msgInner') return inner;
+  return null;
+}}
+{_function_body(UI_JS, "_prepareLiveAnchorScrollRebuildGuard")}
+const snapshot = {{
+  top: 4400,
+  bottom: 0,
+  scrollHeight: 5500,
+  pinned: true,
+  userUnpinned: false,
+}};
+const guard = _prepareLiveAnchorScrollRebuildGuard(snapshot);
+// bottomDistance = 5500 - 4400 - 600 = 500 (>250), but the reader is pinned and
+// has not manually unpinned -> must stay pinned, follow-scroll must NOT be skipped.
+assert.strictEqual(guard.readerAwayFromBottom, false);
+assert.strictEqual(guard.release, null);
+assert.strictEqual(_messageUserUnpinned, false);
+assert.strictEqual(_scrollPinned, true);
+assert.strictEqual(snapshot.pinned, true);
+assert.strictEqual(snapshot.userUnpinned, false);
+assert.strictEqual(inner.style.minHeight, '');
+"""
+    subprocess.run(["node", "-e", script], check=True, capture_output=True, text=True)
+
+
 def test_restore_message_scroll_snapshot_keeps_intermediate_distance_state():
     script = f"""
 const assert = require('assert');