Stage 397: PR #2688 — fix: resync virtualized sidebar after scroll clamp

Co-authored-by: ai-ag2026 <ai-ag2026@users.noreply.github.com>

static/sessions.js

@@ -2932,6 +2932,22 @@ function _markSessionListPointerUp(){
   if(_pendingSessionListPayload) _schedulePendingSessionListApply();
 }
 
+let _sessionVirtualResyncRaf = 0;
+function _resyncSessionVirtualWindowAfterRender(list, expectedScrollTop, virtualWindow){
+  if(!list||!virtualWindow||!virtualWindow.virtualized) return;
+  expectedScrollTop=Number(expectedScrollTop)||0;
+  if(expectedScrollTop<=0) return;
+  if(_sessionVirtualResyncRaf) cancelAnimationFrame(_sessionVirtualResyncRaf);
+  _sessionVirtualResyncRaf=requestAnimationFrame(()=>{
+    _sessionVirtualResyncRaf=0;
+    if(_renamingSid) return;
+    const actualScrollTop=Number(list.scrollTop)||0;
+    const tolerance=Math.max(2, Number(virtualWindow.itemHeight||SESSION_VIRTUAL_ROW_HEIGHT)/2);
+    if(Math.abs(actualScrollTop-expectedScrollTop)<=tolerance) return;
+    renderSessionListFromCache();
+  });
+}
+
 function renderSessionListFromCache(){
   // Don't re-render while user is actively renaming a session (would destroy the input)
   if(_renamingSid) return;
@@ -3214,6 +3230,7 @@ function renderSessionListFromCache(){
     // scrollTop drops to 0 — producing a "scroll keeps jumping back" feel
     // when the list scrolls naturally. Fixed for #1669 follow-up.
     list.scrollTop=listScrollTopBeforeRender;
+    _resyncSessionVirtualWindowAfterRender(list, listScrollTopBeforeRender, virtualWindow);
   }
   // Select mode toggle button (only when NOT in select mode)
   if(!_sessionSelectMode){

tests/test_issue500_session_list_virtualization.py

@@ -138,3 +138,55 @@ def test_session_list_only_moves_to_active_when_active_row_is_not_visible():
     assert before_idx < visible_idx < move_idx < final_idx < anchor_idx
     assert "activeIndex:-1" in render_body[before_idx:visible_idx]
     assert "activeIndex:shouldAnchorActive?activeIndex:-1" not in render_body
+
+
+def test_session_list_resyncs_when_browser_clamps_virtual_scroll_restore():
+    """If a hidden/reflowed sidebar rejects restored scrollTop, re-render the visible window."""
+    js = SESSIONS_JS_PATH.read_text(encoding="utf-8")
+    render_start = js.index("function renderSessionListFromCache()")
+    render_end = js.index("async function _handleActiveSessionStorageEvent", render_start)
+    render_body = js[render_start:render_end]
+
+    assert "_resyncSessionVirtualWindowAfterRender(list, listScrollTopBeforeRender, virtualWindow);" in render_body
+
+    source = _extract_func_script(js) + """
+let renderCount = 0;
+let rafCount = 0;
+let _renamingSid = null;
+const SESSION_VIRTUAL_ROW_HEIGHT = 52;
+function requestAnimationFrame(cb){ rafCount += 1; cb(); return rafCount; }
+function cancelAnimationFrame(_id){}
+function renderSessionListFromCache(){ renderCount += 1; }
+const makeHelper = new Function(
+  'requestAnimationFrame',
+  'cancelAnimationFrame',
+  'renderSessionListFromCache',
+  `let _sessionVirtualResyncRaf = 0;
+   let _renamingSid = null;
+   const SESSION_VIRTUAL_ROW_HEIGHT = 52;
+   ${extractFunc('_resyncSessionVirtualWindowAfterRender')}
+   return _resyncSessionVirtualWindowAfterRender;`
+);
+const _resyncSessionVirtualWindowAfterRender = makeHelper(
+  requestAnimationFrame,
+  cancelAnimationFrame,
+  renderSessionListFromCache
+);
+
+_resyncSessionVirtualWindowAfterRender(
+  {scrollTop: 0},
+  52 * 10,
+  {virtualized: true, itemHeight: 52}
+);
+const afterClamp = renderCount;
+_resyncSessionVirtualWindowAfterRender(
+  {scrollTop: 52 * 10},
+  52 * 10,
+  {virtualized: true, itemHeight: 52}
+);
+console.log(JSON.stringify({afterClamp, final: renderCount, rafCount}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["afterClamp"] == 1
+    assert metrics["final"] == 1
+    assert metrics["rafCount"] == 2