Fix pinned stream scroll restoration

static/ui.js

@@ -11332,10 +11332,33 @@ function _captureMessageScrollSnapshot(){
     userUnpinned:_messageUserUnpinned,
   };
 }
+function _restorePinnedMessageScrollSnapshot(snapshot){
+  const el=$('messages');
+  if(!el||!snapshot||snapshot.pinned!==true||snapshot.userUnpinned===true) return false;
+  const maxTop=Math.max(0,el.scrollHeight-el.clientHeight);
+  const bottom=Number(snapshot.bottom);
+  const target=Number.isFinite(bottom)?maxTop-Math.max(0,bottom):maxTop;
+  _programmaticScroll=true;_programmaticScrollSetAt=performance.now();
+  el.scrollTop=Math.max(0,Math.min(target,maxTop));
+  // Sync _lastScrollTop after programmatic restore so sticky-unpin does not false-trigger (#1731).
+  _lastScrollTop=el.scrollTop;_lastMessageClientHeight=el.clientHeight;
+  _messageUserUnpinned=false;
+  _scrollPinned=true;
+  _nearBottomCount=2;
+  if(typeof _deferClearProgrammaticScroll==='function') _deferClearProgrammaticScroll();
+  else requestAnimationFrame(()=>{ setTimeout(()=>{ _programmaticScroll=false; },0); });
+  return true;
+}
 function _restoreMessageScrollSnapshot(snapshot){
   const el=$('messages');
   if(!el||!snapshot) return;
   const maxTop=Math.max(0,el.scrollHeight-el.clientHeight);
+  // If the reader was following the live tail, preserve the tail-relative bottom
+  // distance. Do not semantic-anchor to the first visible row: live Worklog/
+  // activity rebuilds can remount an older top-of-viewport anchor and yank a
+  // pinned streaming transcript upward. Semantic anchors remain for manual
+  // unpinned reading positions below.
+  if(_restorePinnedMessageScrollSnapshot(snapshot)) return;
   let restoredViaAnchor=(snapshot.anchor&&typeof _restoreMessageViewportAnchor==='function')
     ? _restoreMessageViewportAnchor(snapshot.anchor,0)
     : false;
@@ -11395,6 +11418,10 @@ window._fixMobileScrollJank=function _fixMobileScrollJank(){
 function _restoreMessageScrollSnapshotSameFrame(snapshot){
   const el=$('messages');
   if(!el||!snapshot) return;
+  // Same-frame live DOM updates (tool/worklog/activity rows) are the hot path for
+  // streaming. Pinned followers must stay tail-relative here too; restoring the
+  // semantic viewport anchor is only safe for explicitly unpinned readers.
+  if(_restorePinnedMessageScrollSnapshot(snapshot)) return;
   let restoredViaAnchor=(snapshot.anchor&&typeof _restoreMessageViewportAnchor==='function')
     ? _restoreMessageViewportAnchor(snapshot.anchor,0)
     : false;

tests/test_issue4295_midstream_scroll_anchor.py

@@ -86,6 +86,7 @@ function renderMessages(options) {{
 }}
 {_function_body(UI_JS, "_restoreMessageViewportAnchor")}
 {_function_body(UI_JS, "_remountMessageViewportAnchor")}
+{_function_body(UI_JS, "_restorePinnedMessageScrollSnapshot")}
 {_function_body(UI_JS, "_restoreMessageScrollSnapshot")}
 _restoreMessageScrollSnapshot({{
   anchor: {{ rawIdx: 20, topOffset: 15 }},
@@ -235,6 +236,7 @@ function _messageVirtualScrollTopForVisibleIdx() {{ throw new Error('not expecte
 function renderMessages() {{ throw new Error('not expected'); }}
 {_function_body(UI_JS, "_restoreMessageViewportAnchor")}
 {_function_body(UI_JS, "_remountMessageViewportAnchor")}
+{_function_body(UI_JS, "_restorePinnedMessageScrollSnapshot")}
 {_function_body(UI_JS, "_restoreMessageScrollSnapshot")}
 _restoreMessageScrollSnapshot({{
   anchor: null,

tests/test_issue500_message_list_virtualization.py

@@ -506,6 +506,7 @@ eval(extractFunc('_messageSessionIndexForRawIdx'));
 eval(extractFunc('_messageRawIdxForSessionIndex'));
 eval(extractFunc('_messageVirtualScrollTopForVisibleIdx'));
 eval(extractFunc('_remountMessageViewportAnchor'));
+eval(extractFunc('_restorePinnedMessageScrollSnapshot'));
 eval(extractFunc('_restoreMessageScrollSnapshotSameFrame'));
 
 const snapshot = {

tests/test_tars_scroll_reset_regressions.py

@@ -171,6 +171,33 @@ def test_preserve_scroll_restores_unpinned_viewport_after_dom_rebuild():
     assert "_programmaticScroll=true" in restore
 
 
+def test_pinned_preserve_scroll_uses_bottom_distance_before_viewport_anchor():
+    """Pinned/following streams must not remount an older viewport anchor.
+
+    Live Worklog/activity rebuilds call the same snapshot helpers as reader
+    preservation. For pinned followers, the stable invariant is tail-relative
+    bottom distance, not the first visible row. Restoring the semantic anchor in
+    that state can yank a long streaming transcript upward/topward.
+    """
+    helper = _function_body(UI_JS, "function _restorePinnedMessageScrollSnapshot")
+    restore = _function_body(UI_JS, "function _restoreMessageScrollSnapshot")
+    same_frame = _function_body(UI_JS, "function _restoreMessageScrollSnapshotSameFrame")
+
+    assert "snapshot.pinned!==true||snapshot.userUnpinned===true" in helper
+    assert "maxTop-Math.max(0,bottom)" in helper
+    assert "_messageUserUnpinned=false;" in helper
+    assert "_scrollPinned=true;" in helper
+    assert "_nearBottomCount=2;" in helper
+
+    regular_guard_idx = restore.index("_restorePinnedMessageScrollSnapshot(snapshot)")
+    regular_anchor_idx = restore.index("_restoreMessageViewportAnchor(snapshot.anchor,0)")
+    same_guard_idx = same_frame.index("_restorePinnedMessageScrollSnapshot(snapshot)")
+    same_anchor_idx = same_frame.index("_restoreMessageViewportAnchor(snapshot.anchor,0)")
+
+    assert regular_guard_idx < regular_anchor_idx
+    assert same_guard_idx < same_anchor_idx
+
+
 def test_same_session_reload_anchor_uses_absolute_session_message_index():
     assert "function _messageSessionIndexBase()" in UI_JS
     assert "return _messageSessionIndexBase()+n;" in UI_JS