diff --git a/CHANGELOG.md b/CHANGELOG.md
index 310b90aec..2704aba3e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.423] — 2026-06-15 — Release OJ (virtualize long message transcripts, #500)
+
+### Changed
+
+- **Long chat transcripts are now virtualized for smoother scrolling and rendering.** Sessions with more than 80 messages render only a viewport window (plus a buffer and the most recent 50 messages always-rendered) instead of the entire transcript, with spacer elements preserving scroll geometry. Shorter sessions are unchanged. This removes the jank and slow rebuilds that long (hundreds-of-turn) sessions previously hit. Scroll restoration, jump-to-question, live streaming, and session-switch all preserve their behavior. Note: the browser's native in-page find (Ctrl+F) only matches messages currently in the rendered window on very long transcripts. (#500)
+
 ## [v0.51.422] — 2026-06-14 — Release OI (optional Todos tab in workspace panel, #3564)
 
 ### Added
diff --git a/static/sessions.js b/static/sessions.js
index 98fd6ca2e..118f531d9 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -2393,6 +2393,10 @@ async function _loadOlderMessages() {
     // Use $('messages') — the scrollable container (#msgInner is not scrollable).
     const container = $('messages');
     const prevScrollH = container ? container.scrollHeight : 0;
+    const oldTop = container ? container.scrollTop : 0;
+    const viewportAnchor = (container && typeof _captureMessageViewportAnchor === 'function')
+      ? _captureMessageViewportAnchor()
+      : null;
     // Carry forward ephemeral turn fields (_turnUsage/_turnDuration/_turnTps/
     // _gatewayRouting/_statusCard/_anchor_stream_id) before the wholesale replace so the badge
     // does not briefly appear and disappear during older-message expansion.
@@ -2404,28 +2408,42 @@ async function _loadOlderMessages() {
     // renderMessages() windows long transcripts from the end. If we do not
     // expand that window before rendering, the newly prepended page stays
     // hidden and the "hidden" counter rises while the viewport appears stuck.
-    // Count roughly by the same visible-message rules used by renderMessages().
+    // Count by the same visible-message rules used by renderMessages(); the
+    // virtual fallback below uses this as a pixel-height prefix length.
     const addedRenderable = olderMsgs.filter(m=>{
+      if(typeof _messageIsRenderable==='function') return _messageIsRenderable(m);
       if(!m||!m.role||m.role==='tool') return false;
       if(typeof _isContextCompactionMessage==='function'&&_isContextCompactionMessage(m)) return false;
       if(typeof _isPreservedCompressionTaskListMessage==='function'&&_isPreservedCompressionTaskListMessage(m)) return false;
+      if(typeof _isRecoveryControlMessage==='function'&&_isRecoveryControlMessage(m)) return false;
       const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
       const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-      return !!(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||(typeof _messageHasReasoningPayload==='function'&&_messageHasReasoningPayload(m)))));
+      const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;
+      return !!(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||hasPartialTc||(typeof _messageHasReasoningPayload==='function'&&_messageHasReasoningPayload(m))||(typeof _assistantMessageHasVisibleContent==='function'&&_assistantMessageHasVisibleContent(m)))));
     }).length;
     _messageRenderWindowSize=_currentMessageRenderWindowSize()+Math.max(addedRenderable, MESSAGE_RENDER_WINDOW_DEFAULT);
     _messagesTruncated = !!responseSession._messages_truncated;
     _oldestIdx = responseSession._messages_offset || 0;
     renderMessages({ preserveScroll: true });
     if (container) {
-      // Prepending older messages must not teleport the reader. Preserve the
-      // currently visible viewport by adding the inserted height to scrollTop.
-      const oldTop = container.scrollTop;
-      const newScrollH = container.scrollHeight;
-      const addedHeight = Math.max(0, newScrollH - prevScrollH);
-      _programmaticScroll = true;
-      container.scrollTop = oldTop + addedHeight;
-      requestAnimationFrame(()=>{ _programmaticScroll = false; });
+      // Prepending older messages must not teleport the reader. Anchor to the
+      // first visible rendered row and restore that row's top offset after the
+      // prepend so synthetic virtual spacer heights cannot skew the delta.
+      const restoredViaAnchor = (viewportAnchor && typeof _restoreMessageViewportAnchor === 'function')
+        ? _restoreMessageViewportAnchor(viewportAnchor, olderMsgs.length)
+        : false;
+      if (!restoredViaAnchor) {
+        const virtualAddedHeight = (typeof _messageVirtualPrependedHeightDelta === 'function')
+          ? _messageVirtualPrependedHeightDelta(addedRenderable)
+          : null;
+        const newScrollH = container.scrollHeight;
+        const addedHeight = Number.isFinite(virtualAddedHeight)
+          ? virtualAddedHeight
+          : Math.max(0, newScrollH - prevScrollH);
+        _programmaticScroll = true;
+        container.scrollTop = oldTop + addedHeight;
+        requestAnimationFrame(()=>{ _programmaticScroll = false; });
+      }
     }
     _scrollPinned = false;
   } catch(e) {
diff --git a/static/style.css b/static/style.css
index afa99bac2..201d50dff 100644
--- a/static/style.css
+++ b/static/style.css
@@ -5012,6 +5012,7 @@ main.main > #mainPlugin{display:none;}
 .load-older-indicator:hover{opacity:.75;}
 .message-window-load-earlier{align-self:center;margin:4px auto 14px;padding:7px 14px;border:1px solid var(--border);border-radius:999px;background:var(--surface);box-shadow:var(--shadow-sm);}
 .message-window-load-earlier:hover{background:var(--hover-bg);border-color:var(--accent);}
+.message-virtual-spacer{width:100%;pointer-events:none;}
 .msg-date-sep {
   display: flex; align-items: center; gap: 10px;
   margin: 22px 0 10px; padding: 0 var(--msg-rail);
diff --git a/static/ui.js b/static/ui.js
index 1883e8a34..862301b1e 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -374,8 +374,21 @@ function _statusCardHtml(card){
 }
 
 const MESSAGE_RENDER_WINDOW_DEFAULT=50;
+const MESSAGE_VIRTUAL_THRESHOLD_ROWS=80;
+const MESSAGE_VIRTUAL_BUFFER_PX=900;
+const MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT=140;
+const MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS=2;
 let _messageRenderWindowSid=null;
 let _messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT;
+let _messageVirtualHeightCache=[];
+let _messageVirtualHeightCacheEntries=[];
+let _messageVirtualHeightCacheLen=0;
+let _messageVirtualHeightCacheSrc=null;
+let _messageVirtualEstimatedRowHeight=MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT;
+let _messageVirtualScrollRaf=0;
+let _messageVirtualWindowKey='';
+let _messageVirtualMeasurementCycleKey='';
+let _messageVirtualMeasurementRetryCount=0;
 // Cached visWithIdx array — invalidated when S.messages.length changes.
 let _visWithIdxCache=null;
 let _visWithIdxCacheLen=0;
@@ -385,11 +398,361 @@ function clearVisibleMessageRowCache(){
   _visWithIdxCacheLen=0;
   _visWithIdxCacheSrc=null;
 }
+function _clearMessageVirtualHeightCache(){
+  _messageVirtualHeightCache=[];
+  _messageVirtualHeightCacheEntries=[];
+  _messageVirtualHeightCacheLen=0;
+  _messageVirtualHeightCacheSrc=null;
+  _messageVirtualEstimatedRowHeight=MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT;
+  _messageVirtualWindowKey='';
+  _messageVirtualMeasurementCycleKey='';
+  _messageVirtualMeasurementRetryCount=0;
+}
 function _resetMessageRenderWindow(sid){
   _messageRenderWindowSid=sid||null;
   _messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT;
+  _cancelMessageVirtualizedRender();
   _clearRenderCache();
   clearVisibleMessageRowCache();
+  _clearMessageVirtualHeightCache();
+}
+function _cancelMessageVirtualizedRender(){
+  if(_messageVirtualScrollRaf){
+    cancelAnimationFrame(_messageVirtualScrollRaf);
+    _messageVirtualScrollRaf=0;
+  }
+}
+function _messageIsRenderable(m){
+  if(!m||!m.role||m.role==='tool') return false;
+  if(_isContextCompactionMessage(m)||_isPreservedCompressionTaskListMessage(m)) return false;
+  if(_isRecoveryControlMessage(m)) return false;
+  const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
+  const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
+  const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;
+  const hasReasoningAnchor=hasTc||hasTu||_messageHasReasoningPayload(m);
+  const hasAssistantVisibleAnchor=hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m);
+  return !!(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasReasoningAnchor||hasAssistantVisibleAnchor)));
+}
+function _getVisibleMessagesWithIdx(){
+  if(!_visWithIdxCache || _visWithIdxCacheLen !== S.messages.length || _visWithIdxCacheSrc !== S.messages){
+    const rebuilt=[];
+    let rawIdx=0;
+    for(const m of (S.messages||[])){
+      if(_messageIsRenderable(m)) rebuilt.push({m,rawIdx});
+      rawIdx++;
+    }
+    _visWithIdxCache=rebuilt;
+    _visWithIdxCacheLen=S.messages.length;
+    _visWithIdxCacheSrc=S.messages;
+  }
+  return _visWithIdxCache;
+}
+function _messageVirtualWindow(opts){
+  const total=Math.max(0, Number(opts&&opts.total)||0);
+  const threshold=Math.max(1, Number(opts&&opts.threshold)||MESSAGE_VIRTUAL_THRESHOLD_ROWS);
+  const defaultHeight=Math.max(1, Number(opts&&opts.defaultHeight)||MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT);
+  const bufferPx=Math.max(0, Number(opts&&opts.bufferPx)||MESSAGE_VIRTUAL_BUFFER_PX);
+  const viewportHeight=Math.max(defaultHeight, Number(opts&&opts.viewportHeight)||defaultHeight*6);
+  const keepTailCount=Math.max(0, Number(opts&&opts.keepTailCount)||0);
+  const tailStart=Math.max(0, total-keepTailCount);
+  const heights=Array.isArray(opts&&opts.heights)?opts.heights:[];
+  const rowHeightFor=(idx)=>{
+    const cached=Number(heights[idx]);
+    return Number.isFinite(cached)&&cached>0?cached:defaultHeight;
+  };
+  if(total<=Math.max(threshold, keepTailCount)){
+    return {virtualized:false,start:0,end:total,topPad:0,bottomPad:0,total,tailStart};
+  }
+  const scrollTop=Math.max(0, Number(opts&&opts.scrollTop)||0);
+  const targetTop=Math.max(0, scrollTop-bufferPx);
+  const targetBottom=scrollTop+viewportHeight+bufferPx;
+  let start=0;
+  let offset=0;
+  while(start<tailStart&&offset+rowHeightFor(start)<=targetTop){
+    offset+=rowHeightFor(start);
+    start++;
+  }
+  if(start>=tailStart){
+    return {virtualized:true,start:tailStart,end:tailStart,topPad:offset,bottomPad:0,total,tailStart};
+  }
+  let end=start;
+  let cursor=offset;
+  while(end<tailStart&&cursor<targetBottom){
+    cursor+=rowHeightFor(end);
+    end++;
+  }
+  if(end<=start) end=Math.min(total, start+1);
+  let bottomPad=0;
+  for(let i=end;i<tailStart;i++) bottomPad+=rowHeightFor(i);
+  return {
+    virtualized:true,
+    start,
+    end,
+    topPad:offset,
+    bottomPad,
+    total,
+    tailStart,
+  };
+}
+function _messageVirtualSpacer(height, where){
+  const spacer=document.createElement('div');
+  spacer.className='message-virtual-spacer';
+  spacer.dataset.virtualSpacer=where||'gap';
+  spacer.setAttribute('aria-hidden','true');
+  spacer.style.height=Math.max(0,Math.round(height||0))+'px';
+  spacer.style.flex='0 0 auto';
+  return spacer;
+}
+function _messageVirtualWindowKeyFor(windowMetrics){
+  if(!windowMetrics) return '';
+  return [
+    windowMetrics.virtualized?1:0,
+    windowMetrics.start,
+    windowMetrics.end,
+    Math.round(windowMetrics.topPad||0),
+    Math.round(windowMetrics.bottomPad||0),
+    windowMetrics.tailStart||0,
+  ].join(':');
+}
+function _messageVirtualMeasurementCycleKeyFor(windowMetrics){
+  if(!windowMetrics) return '';
+  return [
+    windowMetrics.virtualized?1:0,
+    windowMetrics.start,
+    windowMetrics.end,
+    windowMetrics.tailStart||0,
+  ].join(':');
+}
+function _scheduleMessageVirtualMeasurementRefresh(windowMetrics){
+  const cycleKey=_messageVirtualMeasurementCycleKeyFor(windowMetrics);
+  if(_messageVirtualMeasurementCycleKey!==cycleKey){
+    _messageVirtualMeasurementCycleKey=cycleKey;
+    _messageVirtualMeasurementRetryCount=0;
+  }
+  if(_messageVirtualMeasurementRetryCount>=MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS) return;
+  _messageVirtualMeasurementRetryCount++;
+  requestAnimationFrame(()=>{ _scheduleMessageVirtualizedRender(true); });
+}
+function _markMessageVirtualMeasurementsSettled(windowMetrics){
+  _messageVirtualMeasurementCycleKey=_messageVirtualMeasurementCycleKeyFor(windowMetrics);
+  _messageVirtualMeasurementRetryCount=0;
+}
+function _messageVirtualHeightEntryMatches(previousEntry, nextEntry){
+  return !!(
+    previousEntry&&nextEntry&&
+    previousEntry.m===nextEntry.m
+  );
+}
+function _messageVirtualHeightPrefixEntryMatches(previousEntry, nextEntry){
+  return !!(
+    previousEntry&&nextEntry&&
+    previousEntry.rawIdx===nextEntry.rawIdx&&
+    _messageVirtualHeightEntryMatches(previousEntry, nextEntry)
+  );
+}
+function _syncMessageVirtualHeightCache(visWithIdx){
+  const nextEntries=Array.isArray(visWithIdx)
+    ? visWithIdx.map(entry=>entry?{rawIdx:entry.rawIdx,m:entry.m}:entry)
+    : [];
+  if(
+    _messageVirtualHeightCacheLen===S.messages.length &&
+    _messageVirtualHeightCacheSrc===S.messages &&
+    _messageVirtualHeightCacheEntries.length===nextEntries.length
+  ) return;
+  const previousEntries=Array.isArray(_messageVirtualHeightCacheEntries)?_messageVirtualHeightCacheEntries:[];
+  const previousHeights=Array.isArray(_messageVirtualHeightCache)?_messageVirtualHeightCache.slice():[];
+  let nextHeights=null;
+  if(!previousEntries.length){
+    nextHeights=new Array(nextEntries.length);
+  }else if(!nextEntries.length){
+    _clearMessageVirtualHeightCache();
+    _messageVirtualHeightCacheLen=S.messages.length;
+    _messageVirtualHeightCacheSrc=S.messages;
+    return;
+  }else{
+    const sharedPrefix=Math.min(previousEntries.length,nextEntries.length);
+    let prefixMatches=true;
+    for(let i=0;i<sharedPrefix;i++){
+      if(!_messageVirtualHeightPrefixEntryMatches(previousEntries[i], nextEntries[i])){
+        prefixMatches=false;
+        break;
+      }
+    }
+    if(prefixMatches){
+      nextHeights=previousHeights.slice(0, sharedPrefix);
+      nextHeights.length=nextEntries.length;
+    }else if(nextEntries.length>=previousEntries.length){
+      const prependedCount=nextEntries.length-previousEntries.length;
+      let suffixMatches=true;
+      for(let i=0;i<previousEntries.length;i++){
+        if(!_messageVirtualHeightEntryMatches(previousEntries[i], nextEntries[i+prependedCount])){
+          suffixMatches=false;
+          break;
+        }
+      }
+      if(suffixMatches){
+        nextHeights=new Array(nextEntries.length);
+        for(let i=0;i<previousEntries.length;i++){
+          nextHeights[prependedCount+i]=previousHeights[i];
+        }
+      }
+    }
+  }
+  if(nextHeights===null){
+    _clearMessageVirtualHeightCache();
+    _messageVirtualHeightCache=new Array(nextEntries.length);
+  }else{
+    _messageVirtualHeightCache=nextHeights;
+    _messageVirtualWindowKey='';
+  }
+  _messageVirtualHeightCacheEntries=nextEntries;
+  _messageVirtualHeightCacheLen=S.messages.length;
+  _messageVirtualHeightCacheSrc=S.messages;
+}
+function _currentMessageVirtualWindow(visWithIdx, keepTailCount){
+  _syncMessageVirtualHeightCache(visWithIdx);
+  const container=$('messages');
+  return _messageVirtualWindow({
+    total:visWithIdx.length,
+    scrollTop:container?container.scrollTop:0,
+    viewportHeight:container?container.clientHeight:(_messageVirtualEstimatedRowHeight*6),
+    heights:_messageVirtualHeightCache,
+    defaultHeight:_messageVirtualEstimatedRowHeight,
+    keepTailCount,
+  });
+}
+function _messageVirtualPrependedHeightDelta(prependedRenderableCount){
+  const count=Math.max(0, Number(prependedRenderableCount)||0);
+  if(count<=0) return null;
+  const visWithIdx=_getVisibleMessagesWithIdx();
+  const virtualWindow=_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount());
+  if(!virtualWindow||!virtualWindow.virtualized) return null;
+  const limit=Math.min(count,_messageVirtualHeightCache.length);
+  let total=0;
+  for(let i=0;i<limit;i++){
+    const cached=Number(_messageVirtualHeightCache[i]);
+    total+=(Number.isFinite(cached)&&cached>0)?cached:_messageVirtualEstimatedRowHeight;
+  }
+  return Math.max(0,Math.round(total));
+}
+function _messageVisibleIndexForRawIdx(rawIdx, visWithIdx){
+  const list=Array.isArray(visWithIdx)?visWithIdx:_getVisibleMessagesWithIdx();
+  for(let i=0;i<list.length;i++){
+    if(list[i]&&list[i].rawIdx===rawIdx) return i;
+  }
+  return -1;
+}
+function _messageVirtualScrollTopForVisibleIdx(visWithIdx, visibleIdx, container){
+  const idx=Math.max(0,Number(visibleIdx)||0);
+  _syncMessageVirtualHeightCache(visWithIdx);
+  const limit=Math.min(idx,_messageVirtualHeightCache.length);
+  let offset=0;
+  for(let i=0;i<limit;i++){
+    const cached=Number(_messageVirtualHeightCache[i]);
+    offset+=(Number.isFinite(cached)&&cached>0)?cached:_messageVirtualEstimatedRowHeight;
+  }
+  const viewport=container?Math.max(0,Number(container.clientHeight)||0):0;
+  return Math.max(0,Math.round(offset-(viewport*0.35)));
+}
+function _messageVirtualKeepTailCount(){
+  return Math.min(_currentMessageRenderWindowSize(), MESSAGE_RENDER_WINDOW_DEFAULT);
+}
+function _captureMessageViewportAnchor(){
+  const container=$('messages');
+  if(!container) return null;
+  const containerRect=container.getBoundingClientRect();
+  const rows=Array.from(container.querySelectorAll('[data-msg-idx]'));
+  for(const row of rows){
+    const rawIdx=Number(row&&row.dataset&&row.dataset.msgIdx);
+    if(!Number.isFinite(rawIdx)) continue;
+    const rect=row.getBoundingClientRect();
+    if(rect.bottom>containerRect.top+1){
+      return {rawIdx, topOffset:rect.top-containerRect.top};
+    }
+  }
+  return null;
+}
+function _restoreMessageViewportAnchor(anchor, rawIdxDelta){
+  const container=$('messages');
+  if(!container||!anchor) return false;
+  const targetIdx=Number(anchor.rawIdx)+Number(rawIdxDelta||0);
+  if(!Number.isFinite(targetIdx)) return false;
+  const row=container.querySelector(`[data-msg-idx="${targetIdx}"]`);
+  if(!row) return false;
+  const containerRect=container.getBoundingClientRect();
+  const rect=row.getBoundingClientRect();
+  const targetTop=Number(anchor.topOffset)||0;
+  _programmaticScroll=true;
+  container.scrollTop+=(rect.top-containerRect.top)-targetTop;
+  requestAnimationFrame(()=>{ _programmaticScroll=false; });
+  return true;
+}
+function _measureMessageVirtualRow(inner, entry){
+  if(!inner||!entry) return 0;
+  const primary=inner.querySelector(`[data-msg-idx="${entry.rawIdx}"]`);
+  if(!primary) return 0;
+  let totalHeight=Math.max(0, primary.getBoundingClientRect().height||0);
+  if(primary.classList.contains('assistant-segment')){
+    let sibling=primary.nextElementSibling;
+    while(sibling){
+      if(sibling.hasAttribute('data-msg-idx')) break;
+      if(!(sibling.matches&&sibling.matches('.tool-call-group,.tool-card-row,.agent-activity-thinking,.thinking-card-row'))) break;
+      totalHeight+=Math.max(0, sibling.getBoundingClientRect().height||0);
+      sibling=sibling.nextElementSibling;
+    }
+  }
+  return totalHeight;
+}
+function _updateMessageVirtualMeasurements(renderVisWithIdx, renderVisibleIdxs, virtualWindow){
+  const inner=$('msgInner');
+  if(!inner||!virtualWindow||!virtualWindow.virtualized||!renderVisWithIdx.length) return;
+  let changed=false;
+  let measuredCount=0;
+  let measuredTotal=0;
+  for(let vi=0;vi<renderVisWithIdx.length;vi++){
+    const entry=renderVisWithIdx[vi];
+    if(!entry) continue;
+    const totalHeight=_measureMessageVirtualRow(inner, entry);
+    if(totalHeight<=0) continue;
+    const visibleIdx=Number(renderVisibleIdxs&&renderVisibleIdxs[vi]);
+    if(!Number.isFinite(visibleIdx)) continue;
+    if(Math.abs((Number(_messageVirtualHeightCache[visibleIdx])||0)-totalHeight)>1){
+      _messageVirtualHeightCache[visibleIdx]=totalHeight;
+      changed=true;
+    }
+    measuredTotal+=totalHeight;
+    measuredCount++;
+  }
+  if(measuredCount>0){
+    _messageVirtualEstimatedRowHeight=Math.max(60, Math.round(measuredTotal/measuredCount));
+  }
+  if(changed){
+    _scheduleMessageVirtualMeasurementRefresh(virtualWindow);
+  }else{
+    _markMessageVirtualMeasurementsSettled(virtualWindow);
+  }
+}
+function _scheduleMessageVirtualizedRender(force){
+  const container=$('messages');
+  const inner=$('msgInner');
+  if(!container||!inner) return;
+  const visWithIdx=_getVisibleMessagesWithIdx();
+  const virtualWindow=_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount());
+  const nextKey=_messageVirtualWindowKeyFor(virtualWindow);
+  if(!force&&nextKey===_messageVirtualWindowKey) return;
+  if(!virtualWindow.virtualized){
+    _messageVirtualWindowKey=nextKey;
+    return;
+  }
+  if(_messageVirtualScrollRaf) return;
+  _messageVirtualScrollRaf=requestAnimationFrame(()=>{
+    _messageVirtualScrollRaf=0;
+    const liveVisWithIdx=_getVisibleMessagesWithIdx();
+    const liveWindow=_currentMessageVirtualWindow(liveVisWithIdx,_messageVirtualKeepTailCount());
+    const liveKey=_messageVirtualWindowKeyFor(liveWindow);
+    if(!force&&liveKey===_messageVirtualWindowKey) return;
+    renderMessages({ preserveScroll:true });
+  });
 }
 
 // ── renderMd / _renderUserFencedBlocks cache ──────────────────────────────
@@ -425,16 +788,7 @@ function _currentMessageRenderWindowSize(){
   );
 }
 function _messageRenderableMessageCount(){
-  let count=0;
-  for(const m of (S.messages||[])){
-    if(!m||!m.role||m.role==='tool') continue;
-    if(_isContextCompactionMessage(m)||_isPreservedCompressionTaskListMessage(m)) continue;
-    if(_isRecoveryControlMessage(m)) continue;
-    const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
-    const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-    if(msgContent(m)||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)))) count++;
-  }
-  return count;
+  return _getVisibleMessagesWithIdx().length;
 }
 function _messageHiddenBeforeCount(){
   return Math.max(0,_messageRenderableMessageCount()-_currentMessageRenderWindowSize());
@@ -446,22 +800,9 @@ function _wireMessageWindowLoadEarlierButton(){
   const indicator=$('loadOlderIndicator');
   if(!indicator) return;
   indicator.onclick=()=>{
-    if(_messageHiddenBeforeCount()>0) _showEarlierRenderedMessages();
-    else if(typeof _loadOlderMessages==='function') _loadOlderMessages();
+    if(typeof _loadOlderMessages==='function') _loadOlderMessages();
   };
 }
-function _showEarlierRenderedMessages(){
-  const container=$('messages');
-  const prevScrollH=container?container.scrollHeight:0;
-  const prevScrollTop=container?container.scrollTop:0;
-  _messageRenderWindowSize=_currentMessageRenderWindowSize()+MESSAGE_RENDER_WINDOW_DEFAULT;
-  renderMessages();
-  if(container){
-    const newScrollH=container.scrollHeight;
-    container.scrollTop=prevScrollTop+(newScrollH-prevScrollH);
-  }
-  _scrollPinned=false;
-}
 function _isSessionJumpButtonsEnabled(){
   return window._sessionJumpButtonsEnabled===true;
 }
@@ -498,6 +839,8 @@ async function jumpToSessionStart(){
       if(typeof _ensureAllMessagesLoaded==='function') await _ensureAllMessagesLoaded();
     }
     _messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount());
+    container.scrollTop=0;
+    _messageVirtualWindowKey='';
     // During streaming, skip renderMessages — it rebuilds the DOM but tool card
     // insertion is blocked by !S.busy, losing Activity until "done" fires.
     if(!(S.busy||S.activeStreamId)){
@@ -560,8 +903,29 @@ async function jumpToTurnQuestion(questionRawIdx, assistantRawIdx){
     return true;
   };
   if(scrollToTarget()) return;
+  const visWithIdx=_getVisibleMessagesWithIdx();
+  const visibleIdx=_messageVisibleIndexForRawIdx(questionRawIdx, visWithIdx);
+  if(visibleIdx>=0){
+    _scrollPinned=false;
+    _messageUserUnpinned=true;
+    _programmaticScroll=true;
+    container.scrollTop=_messageVirtualScrollTopForVisibleIdx(visWithIdx, visibleIdx, container);
+    _messageVirtualWindowKey='';
+    renderMessages({ preserveScroll:true });
+    requestAnimationFrame(()=>{
+      if(!scrollToTarget()&&_messageHiddenBeforeCount()>0){
+        _messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount());
+        _messageVirtualWindowKey='';
+        renderMessages({ preserveScroll:true });
+        requestAnimationFrame(scrollToTarget);
+      }
+      requestAnimationFrame(()=>{ _programmaticScroll=false; });
+    });
+    return;
+  }
   if(_messageHiddenBeforeCount()>0){
     _messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount());
+    _messageVirtualWindowKey='';
     renderMessages({ preserveScroll:true });
     requestAnimationFrame(scrollToTarget);
   }
@@ -2727,6 +3091,7 @@ if(typeof window!=='undefined'){
       const showBottomButton=!_scrollPinned && el.scrollHeight-top-el.clientHeight>80;
       _syncScrollToBottomCue(showBottomButton,{newMessage:_newMessageCueVisible});
       if(typeof _updateSessionStartJumpButton==='function') _updateSessionStartJumpButton();
+      _scheduleMessageVirtualizedRender();
       // Prefetch older messages before the reader hits the hard top. Prepending
       // then preserving scrollTop is seamless only if there is runway left for
       // the user's continued upward wheel/touch movement.
@@ -8615,6 +8980,7 @@ function clearMessageRenderCache(){
   _sessionHtmlCache.clear();
   _sessionHtmlCacheSid=null;
   clearVisibleMessageRowCache();
+  _clearMessageVirtualHeightCache();
 }
 
 function _messageRenderCacheSignature(){
@@ -8938,13 +9304,31 @@ function renderMessages(options){
   // renderMessages() in this window. Keep the existing loading placeholder.
   if(_loadingSessionId===sid&&msgCount===0&&inner) return;
   if(sid!==_messageRenderWindowSid) _resetMessageRenderWindow(sid);
-  const renderWindowSize=_currentMessageRenderWindowSize();
   let cachedRenderSignature=null;
   const hasTransientTranscriptUi=!!(
     (window._compressionUi&&(!window._compressionUi.sessionId||window._compressionUi.sessionId===sid)) ||
     (window._handoffUi&&(!window._handoffUi.sessionId||window._handoffUi.sessionId===sid))
   );
 
+  const preservedCompressionTaskMessages=_latestPreservedCompressionTaskListMessages(S.messages);
+  const visWithIdx=_getVisibleMessagesWithIdx();
+  $('emptyState').style.display=(visWithIdx.length||preservedCompressionTaskMessages.length)?'none':'';
+  const virtualWindow=_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount());
+  const renderWindowKey=_messageVirtualWindowKeyFor(virtualWindow);
+  const windowStart=virtualWindow.start;
+  const windowEnd=virtualWindow.end;
+  const renderHeadVisWithIdx=visWithIdx.slice(windowStart, windowEnd);
+  const renderTailStart=virtualWindow.virtualized?Math.max(windowEnd, virtualWindow.tailStart):windowEnd;
+  const renderTailVisWithIdx=virtualWindow.virtualized&&renderTailStart<visWithIdx.length
+    ? visWithIdx.slice(renderTailStart)
+    : [];
+  const renderVisWithIdx=renderHeadVisWithIdx.concat(renderTailVisWithIdx);
+  const renderVisibleIdxs=[
+    ...renderHeadVisWithIdx.map((_,idx)=>windowStart+idx),
+    ...renderTailVisWithIdx.map((_,idx)=>renderTailStart+idx),
+  ];
+  const headRenderCount=renderHeadVisWithIdx.length;
+
   // Fast path: switching back to a previously rendered session with same count.
   // Guard: sid !== _sessionHtmlCacheSid ensures in-session updates (edits,
   // new messages, tool_complete) always get a fresh rebuild.
@@ -8957,21 +9341,38 @@ function renderMessages(options){
     const renderSignature=_messageRenderCacheSignature();
     cachedRenderSignature=renderSignature;
     const cached=_sessionHtmlCache.get(sid);
-    if(cached&&cached.msgCount===msgCount&&cached.renderWindowSize===renderWindowSize&&cached.signature===renderSignature){
+    if(cached&&cached.msgCount===msgCount&&cached.renderWindowKey===renderWindowKey&&cached.signature===renderSignature){
       inner.innerHTML=cached.html;
+      _messageVirtualWindowKey=renderWindowKey;
       _sessionHtmlCacheSid=sid;
       _rehydrateTransparentStreamDom(inner);
       _wireMessageWindowLoadEarlierButton();
       if(typeof _applySessionNavigationPrefs==='function') _applySessionNavigationPrefs();
       _scrollAfterMessageRender(preserveScroll, scrollSnapshot);
+      _updateMessageVirtualMeasurements(renderVisWithIdx, renderVisibleIdxs, virtualWindow);
       requestAnimationFrame(()=>postProcessRenderedMessages(inner));
       if(typeof _initMediaPlaybackObserver==='function') _initMediaPlaybackObserver();
       if(typeof loadTodos==='function'&&document.getElementById('panelTodos')&&document.getElementById('panelTodos').classList.contains('active')){loadTodos();}
       return;
     }
   }
-  const worklogDetailDisclosureState=_captureWorklogDetailDisclosureState(inner);
-
+  // Mid-stream flicker fix (#3877): when a renderMessages() rebuild is reached
+  // while THIS session is actively streaming (e.g. the clarify-response echo at
+  // messages.js, or a CLI-import refresh), the `inner.innerHTML=''` below detaches
+  // the live `#liveAssistantTurn` node — and the smd parser keeps writing into
+  // that now-orphaned node, so the streamed text vanishes until the next stream
+  // event rebuilds the turn ("disappears, then reappears"). Capture the live
+  // turn's actual DOM node (not its HTML — the parser holds a live reference into
+  // it) so it can be re-attached after the rebuild, keeping the parser target
+  // connected and the streamed text visible. Only for the streaming session's own
+  // live turn; never affects settled transcripts.
+  let _preservedLiveTurn=null;
+  if(sid&&INFLIGHT[sid]){
+    const _lt=document.getElementById('liveAssistantTurn');
+    if(_lt&&(!_lt.dataset||!_lt.dataset.sessionId||_lt.dataset.sessionId===sid)){
+      _preservedLiveTurn=_lt;
+    }
+  }
   const compressionState=(()=>{
     let compressionState=_compressionStateForCurrentSession();
     if(!S.busy && compressionState && compressionState.automatic){
@@ -8994,41 +9395,7 @@ function renderMessages(options){
   const sessionCompressionSummary=(
     S.session && typeof S.session.compression_anchor_summary==='string'
   ) ? S.session.compression_anchor_summary.trim() : '';
-  const preservedCompressionTaskMessages=_latestPreservedCompressionTaskListMessages(S.messages);
-  const vis=S.messages.filter(m=>{
-    if(!m||!m.role||m.role==='tool')return false;
-    if(_isContextCompactionMessage(m)) return false;
-    if(_isPreservedCompressionTaskListMessage(m)) return false;
-    if(_isRecoveryControlMessage(m)) return false;
-    if(m.role==='assistant'){
-      const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
-      const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-      const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;
-      if(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)) return true;
-      if(_assistantMessageHasVisibleContent(m)) return true;
-      const visibleText=_isAssistantEmptyPlaceholderContent(m,msgContent(m))?'':msgContent(m);
-      return m._statusCard||visibleText||m.attachments?.length;
-    }
-    return m._statusCard||msgContent(m)||m.attachments?.length;
-  });
-  $('emptyState').style.display=(vis.length||preservedCompressionTaskMessages.length)?'none':'';
-  // Mid-stream flicker fix (#3877): when a renderMessages() rebuild is reached
-  // while THIS session is actively streaming (e.g. the clarify-response echo at
-  // messages.js, or a CLI-import refresh), the `inner.innerHTML=''` below detaches
-  // the live `#liveAssistantTurn` node — and the smd parser keeps writing into
-  // that now-orphaned node, so the streamed text vanishes until the next stream
-  // event rebuilds the turn ("disappears, then reappears"). Capture the live
-  // turn's actual DOM node (not its HTML — the parser holds a live reference into
-  // it) so it can be re-attached after the rebuild, keeping the parser target
-  // connected and the streamed text visible. Only for the streaming session's own
-  // live turn; never affects settled transcripts.
-  let _preservedLiveTurn=null;
-  if(sid&&INFLIGHT[sid]){
-    const _lt=document.getElementById('liveAssistantTurn');
-    if(_lt&&(!_lt.dataset||!_lt.dataset.sessionId||_lt.dataset.sessionId===sid)){
-      _preservedLiveTurn=_lt;
-    }
-  }
+  const worklogDetailDisclosureState=_captureWorklogDetailDisclosureState(inner);
   inner.innerHTML='';
   const compressionNode=compressionState?_compressionCardsNode(compressionState):null;
   const {message:referenceMessage, rawIdx:referenceMessageRawIdx}=_latestCompressionReferenceMessage(
@@ -9042,29 +9409,6 @@ function renderMessages(options){
     ? (()=>{const row=document.createElement('div');row.innerHTML=`<div class="compression-turn"><div class="compression-turn-blocks">${_compressionReferenceCardHtml(referenceText,false)}${_preservedCompressionTaskListCardsHtml(preservedCompressionTaskMessages)}</div></div>`;return row.firstElementChild;})()
     : null;
   let preservedCompressionTaskCardsAttached=!!referenceNode;
-  // Cache visWithIdx so expanding the render window (Load earlier) doesn't
-  // re-scan S.messages from scratch.  Invalidate only when the message array
-  // length changes — i.e. new messages arrived or session was truncated.
-  if(!_visWithIdxCache || _visWithIdxCacheLen !== S.messages.length || _visWithIdxCacheSrc !== S.messages){
-    const rebuilt=[];
-    let ri=0;
-    for(const m of S.messages){
-      if(!m||!m.role||m.role==='tool'){ri++;continue;}
-      if(_isContextCompactionMessage(m)){ri++;continue;}
-      if(_isPreservedCompressionTaskListMessage(m)){ri++;continue;}
-      if(_isRecoveryControlMessage(m)){ri++;continue;}
-      const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;
-      const hasTu=Array.isArray(m.content)&&m.content.some(p=>p&&p.type==='tool_use');
-      const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;
-      const visibleText=_isAssistantEmptyPlaceholderContent(m,msgContent(m))?'':msgContent(m);
-      if(visibleText||m._statusCard||m.attachments?.length||(m.role==='assistant'&&(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)))) rebuilt.push({m,rawIdx:ri});
-      ri++;
-    }
-    _visWithIdxCache=rebuilt;
-    _visWithIdxCacheLen=S.messages.length;
-    _visWithIdxCacheSrc=S.messages;
-  }
-  const visWithIdx=_visWithIdxCache;
   const preservedCompressionRawIdxs=[];
   let rawIdx=0;
   for(const m of S.messages){
@@ -9072,33 +9416,23 @@ function renderMessages(options){
     if(_isPreservedCompressionTaskListMessage(m)){preservedCompressionRawIdxs.push(rawIdx);rawIdx++;continue;}
     rawIdx++;
   }
-  // Show a top affordance when earlier transcript content exists either in
-  // memory (DOM windowing) or on the server (paginated session fetch).
-  // Prefer expanding the local render window first so a fully loaded long
-  // session can reduce DOM nodes without losing in-memory transcript data.
-  const windowStart=Math.max(0, visWithIdx.length-renderWindowSize);
-  const hiddenBeforeCount=windowStart;
-  const renderVisWithIdx=visWithIdx.slice(windowStart);
   const firstRenderedRawIdx=renderVisWithIdx.length?renderVisWithIdx[0].rawIdx:Infinity;
   const assistantTurnFinalVisibleContentByRawIdx=_assistantTurnFinalVisibleContentMap(visWithIdx);
   const assistantTurnVisibleContentByRawIdx=_assistantTurnVisibleContentMap(visWithIdx);
   const hasServerOlder=!!(typeof _messagesTruncated!=='undefined' && _messagesTruncated && S.messages.length>0);
   const serverOlderCount=hasServerOlder&&Number.isFinite(Number(_oldestIdx))?Math.max(0,Number(_oldestIdx)):0;
   if(typeof _applySessionNavigationPrefs==='function') _applySessionNavigationPrefs();
-  if(hiddenBeforeCount>0 || hasServerOlder){
+  if(virtualWindow.virtualized&&virtualWindow.topPad>0){
+    inner.appendChild(_messageVirtualSpacer(virtualWindow.topPad,'before'));
+  }
+  if(hasServerOlder){
     const indicator=document.createElement('button');
     indicator.type='button';
     indicator.id='loadOlderIndicator';
     indicator.className='load-older-indicator message-window-load-earlier';
-    indicator.textContent=hiddenBeforeCount>0
-      ? `Load earlier messages (${hiddenBeforeCount} hidden)`
-      : (serverOlderCount>0
-        ? `Load earlier messages (${serverOlderCount} older)`
-        : (typeof t==='function'?t('load_older_messages'):'Load earlier messages'));
-    indicator.onclick=()=>{
-      if(hiddenBeforeCount>0) _showEarlierRenderedMessages();
-      else if(typeof _loadOlderMessages==='function') _loadOlderMessages();
-    };
+    indicator.textContent=serverOlderCount>0
+      ? `Load earlier messages (${serverOlderCount} older)`
+      : (typeof t==='function'?t('load_older_messages'):'Load earlier messages');
     inner.appendChild(indicator);
     _wireMessageWindowLoadEarlierButton();
   }
@@ -9118,9 +9452,21 @@ function renderMessages(options){
   );
   let insertionAnchor=null;
   if(typeof insertionAnchorFull==='number'){
-    if(insertionAnchorFull<windowStart) insertionAnchor=renderVisWithIdx.length?0:null;
-    else if(insertionAnchorFull<windowStart+renderVisWithIdx.length) insertionAnchor=insertionAnchorFull-windowStart;
-    else insertionAnchor=renderVisWithIdx.length?renderVisWithIdx.length-1:null;
+    const hasVirtualRenderGap=renderVisibleIdxs.some((idx,pos)=>idx!==windowStart+pos);
+    if(!hasVirtualRenderGap){
+      if(insertionAnchorFull<windowStart) insertionAnchor=renderVisWithIdx.length?0:null;
+      else if(insertionAnchorFull<windowStart+renderVisWithIdx.length) insertionAnchor=insertionAnchorFull-windowStart;
+      else insertionAnchor=renderVisWithIdx.length?renderVisWithIdx.length-1:null;
+    }else if(renderVisibleIdxs.length){
+      let previousVisibleIdx=-1;
+      for(let i=0;i<renderVisibleIdxs.length;i++){
+        if(renderVisibleIdxs[i]<=insertionAnchorFull) previousVisibleIdx=i;
+        else break;
+      }
+      insertionAnchor=previousVisibleIdx>=0?previousVisibleIdx:0;
+    }else{
+      insertionAnchor=null;
+    }
   }
   let _prevSepKey=null;
   let currentAssistantTurn=null;
@@ -9128,18 +9474,13 @@ function renderMessages(options){
   // full visWithIdx.  The jump-to-question button is only rendered for
   // assistant messages that appear in the current render window anyway.
   const questionRawIdxByAssistantRawIdx=new Map();
-  // Seed lastQuestionRawIdx from hidden messages so the first visible
-  // assistant message still gets a valid jump target even when its
-  // corresponding user message sits just before the render window.
   let lastQuestionRawIdx=-1;
-  for(let i=0;i<windowStart;i++){
-    const role=visWithIdx[i]?.m?.role;
-    if(role==='user') lastQuestionRawIdx=visWithIdx[i].rawIdx;
-  }
-  for(const entry of renderVisWithIdx){
+  const renderedRawIdxs=new Set(renderVisWithIdx.map(e=>e.rawIdx));
+  const renderableRawIdxs=new Set(visWithIdx.map(e=>e.rawIdx));
+  for(const entry of visWithIdx){
     const role=entry&&entry.m&&entry.m.role;
     if(role==='user') lastQuestionRawIdx=entry.rawIdx;
-    else if(role==='assistant') questionRawIdxByAssistantRawIdx.set(entry.rawIdx,lastQuestionRawIdx);
+    else if(role==='assistant'&&renderedRawIdxs.has(entry.rawIdx)) questionRawIdxByAssistantRawIdx.set(entry.rawIdx,lastQuestionRawIdx);
   }
   const assistantRawIdxByQuestionRawIdx=new Map();
   for(const [aIdx,qIdx] of questionRawIdxByAssistantRawIdx){
@@ -9187,7 +9528,6 @@ function renderMessages(options){
   // range.
   const toolCallAssistantIdxs=new Set();
   if(Array.isArray(S.toolCalls)){
-    const renderedRawIdxs=new Set(renderVisWithIdx.map(e=>e.rawIdx));
     for(const tc of S.toolCalls){
       if(!tc) continue;
       const idx=tc.assistant_msg_idx;
@@ -9199,6 +9539,13 @@ function renderMessages(options){
   // Windowed render loop replaces the legacy full loop:
   // for(let vi=0;vi<visWithIdx.length;vi++)
   for(let vi=0;vi<renderVisWithIdx.length;vi++){
+    if(virtualWindow.virtualized&&virtualWindow.bottomPad>0&&vi===headRenderCount){
+      // The virtual gap breaks assistant-turn adjacency. Reset the current
+      // turn before rendering the always-visible tail so assistant segments do
+      // not merge across the spacer boundary.
+      currentAssistantTurn=null;
+      inner.appendChild(_messageVirtualSpacer(virtualWindow.bottomPad,'after'));
+    }
     const {m,rawIdx}=renderVisWithIdx[vi];
     const _tsSep=m._ts||m.timestamp;
     if(_tsSep){
@@ -9708,6 +10055,7 @@ function renderMessages(options){
     for(const tc of (S.toolCalls||[])){
       if(!tc) continue;
       const aIdx=tc.assistant_msg_idx!==undefined?parseInt(tc.assistant_msg_idx):-1;
+      if(virtualWindow.virtualized&&renderableRawIdxs.has(aIdx)&&!renderedRawIdxs.has(aIdx)) continue;
       const segmentSeq=normalizeToken(tc.activitySegmentSeq);
       const burstId=normalizeToken(tc.activityBurstId);
       const key=segmentSeq?`segment:${segmentSeq}`:(burstId?`burst:${burstId}`:`assistant:${aIdx}`);
@@ -9716,6 +10064,7 @@ function renderMessages(options){
       entry.includeAnchorReason=true;
     }
     for(const aIdx of assistantThinking.keys()){
+      if(virtualWindow.virtualized&&renderableRawIdxs.has(aIdx)&&!renderedRawIdxs.has(aIdx)) continue;
       const seg=assistantSegments.get(aIdx);
       const segmentSeq=seg&&seg.getAttribute('data-live-segment-seq')||'';
       const burstId=seg&&seg.getAttribute('data-activity-burst-id')||'';
@@ -9725,6 +10074,7 @@ function renderMessages(options){
     }
     for(const [aIdx,seg] of assistantSegments){
       if(!seg||!seg.classList||!seg.classList.contains('assistant-segment-worklog-source')) continue;
+      if(virtualWindow.virtualized&&renderableRawIdxs.has(aIdx)&&!renderedRawIdxs.has(aIdx)) continue;
       if(!_worklogReasonHtmlFromAnchor(seg)) continue;
       const segmentSeq=seg&&seg.getAttribute('data-live-segment-seq')||'';
       const burstId=seg&&seg.getAttribute('data-activity-burst-id')||'';
@@ -9864,6 +10214,7 @@ function renderMessages(options){
     }
   }
   _restoreWorklogDetailDisclosureState(inner, worklogDetailDisclosureState);
+  _messageVirtualWindowKey=renderWindowKey;
   // Render per-turn duration and optional token usage on assistant messages.
   // Duration stays visible even when token usage is disabled, because it answers
   // the basic "how long did that turn take?" UX question. Only walk rendered
@@ -10104,7 +10455,21 @@ function renderMessages(options){
       }
     }
     const _preservedLen=_liveAssistantSegmentTextLength(_preservedSeg||_preservedLiveTurn);
-    if(_preservedLen>0){
+    // Structural-block counts: a live turn can be AHEAD of S.messages with
+    // Activity/tool/worklog blocks that haven't persisted yet — even with ZERO
+    // streamed text (e.g. an Activity-only turn mid-tool-call). The text-length
+    // gate alone would skip preservation in that case, so a scroll-triggered
+    // rebuild on a long (virtualized) transcript could blink those live-only
+    // blocks for a frame. Also restore when the preserved turn carries more
+    // structure than the rebuilt (lagging-S.messages) turn. (#3714 ship-review)
+    const _structuralCount=(turn)=> turn?turn.querySelectorAll(
+      '[data-live-assistant="1"],.tool-call-group,.tool-card-row,'+
+      '.tool-worklog-group,.live-worklog[data-live-worklog-shell="1"],'+
+      '.wl-reason,.agent-activity-thinking,.thinking-card-row'
+    ).length:0;
+    const _preservedStructure=_structuralCount(_preservedLiveTurn);
+    const _rebuiltStructure=_structuralCount(_rebuilt);
+    if(_preservedLen>0 || _preservedStructure>_rebuiltStructure){
       const _rebuiltLen=_rebuilt?_liveAssistantSegmentTextLength(_rebuiltSeg||_rebuilt):-1;
       if(_rebuiltLen<=_preservedLen){
         // Decide segment-level vs whole-turn restore. Segment-level keeps the
@@ -10117,13 +10482,6 @@ function renderMessages(options){
         // blocks for a frame — so restore the WHOLE preserved turn instead.
         // Otherwise (rebuild has >= the preserved turn's structural blocks) do
         // the precise segment swap so rebuilt-only structure is kept.
-        const _structuralCount=(turn)=> turn?turn.querySelectorAll(
-          '[data-live-assistant="1"],.tool-call-group,.tool-card-row,'+
-          '.tool-worklog-group,.live-worklog[data-live-worklog-shell="1"],'+
-          '.wl-reason,.agent-activity-thinking,.thinking-card-row'
-        ).length:0;
-        const _preservedStructure=_structuralCount(_preservedLiveTurn);
-        const _rebuiltStructure=_structuralCount(_rebuilt);
         if(_rebuilt&&_rebuiltSeg&&_preservedSeg&&_rebuiltStructure>=_preservedStructure){
           // Rebuild is the structural superset — swap only the parser-owned
           // (tail) live segment, keeping rebuilt-only segments / tool groups.
@@ -10163,10 +10521,11 @@ function renderMessages(options){
     // Only cache sessions with <300KB rendered HTML; evict oldest beyond 8 sessions.
     if(_html.length<300_000){
       const renderSignature=cachedRenderSignature===null?_messageRenderCacheSignature():cachedRenderSignature;
-      _sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize,signature:renderSignature});
+      _sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowKey,signature:renderSignature});
       if(_sessionHtmlCache.size>8){_sessionHtmlCache.delete(_sessionHtmlCache.keys().next().value);}
     }
   }
+  _updateMessageVirtualMeasurements(renderVisWithIdx, renderVisibleIdxs, virtualWindow);
 }
 
 function _toolDisplayName(tc){
diff --git a/tests/test_auto_compression_card.py b/tests/test_auto_compression_card.py
index f65a2d0cf..68b900dd2 100644
--- a/tests/test_auto_compression_card.py
+++ b/tests/test_auto_compression_card.py
@@ -589,31 +589,25 @@ def test_settled_transcript_suppresses_context_compaction_reference_cards():
     assert "function _shouldShowSettledCompressionReference" in src
     assert "!_isContextCompactionText(referenceText)" in src
 
-    visible_filter_start = src.find("const vis=S.messages.filter")
-    assert visible_filter_start != -1, "visible message filter not found"
-    visible_filter_end = src.find("$('emptyState')", visible_filter_start)
-    visible_filter = src[visible_filter_start:visible_filter_end]
-    assert "if(_isContextCompactionMessage(m)) return false;" in visible_filter
-
-    vis_idx_start = src.find("for(const m of S.messages)", visible_filter_end)
-    assert vis_idx_start != -1, "raw message index loop not found"
-    vis_idx_end = src.find("let lastUserRawIdx", vis_idx_start)
-    vis_idx_loop = src[vis_idx_start:vis_idx_end]
-    assert "if(_isContextCompactionMessage(m)){ri++;continue;}" in vis_idx_loop
+    helper_start = src.find("function _messageIsRenderable")
+    assert helper_start != -1, "renderable message helper not found"
+    helper_end = src.find("function _getVisibleMessagesWithIdx", helper_start)
+    assert helper_end != -1, "visible message index helper not found after renderable helper"
+    helper = src[helper_start:helper_end]
+    assert "_isContextCompactionMessage(m)||_isPreservedCompressionTaskListMessage(m)" in helper
 
 
 def test_preserved_task_list_skips_normal_visible_message_path():
     src = _read("static/ui.js")
 
-    visible_filter_start = src.find("const vis=S.messages.filter")
-    assert visible_filter_start != -1, "visible message filter not found"
-    visible_filter_end = src.find("$('emptyState')", visible_filter_start)
-    assert visible_filter_end != -1, "empty state update after visible filter not found"
-    visible_filter = src[visible_filter_start:visible_filter_end]
-    assert "if(_isContextCompactionMessage(m)) return false;" in visible_filter
-    assert "if(_isPreservedCompressionTaskListMessage(m)) return false;" in visible_filter
+    helper_start = src.find("function _messageIsRenderable")
+    assert helper_start != -1, "renderable message helper not found"
+    helper_end = src.find("function _getVisibleMessagesWithIdx", helper_start)
+    assert helper_end != -1, "visible message index helper not found after renderable helper"
+    helper = src[helper_start:helper_end]
+    assert "_isContextCompactionMessage(m)||_isPreservedCompressionTaskListMessage(m)" in helper
 
-    vis_idx_start = src.find("for(const m of S.messages)", visible_filter_end)
+    vis_idx_start = src.find("for(const m of S.messages)", helper_end)
     assert vis_idx_start != -1, "raw message index loop not found"
     vis_idx_end = src.find("let lastUserRawIdx", vis_idx_start)
     assert vis_idx_end != -1, "last user index lookup after raw message loop not found"
@@ -675,7 +669,9 @@ def test_compression_anchor_index_is_translated_into_render_window():
     assert "_compressionAnchorIndex(\n    visWithIdx," in block
     assert "insertionAnchorFull<windowStart" in block
     assert "insertionAnchorFull-windowStart" in block
-    assert "windowStart+renderVisWithIdx.length" in block
+    assert "let previousVisibleIdx=-1;" in block
+    assert "if(renderVisibleIdxs[i]<=insertionAnchorFull) previousVisibleIdx=i;" in block
+    assert "insertionAnchor=previousVisibleIdx>=0?previousVisibleIdx:0;" in block
 
 
 def test_reference_message_uses_raw_transcript_position_before_anchor_fallback():
diff --git a/tests/test_issue2246_question_jump.py b/tests/test_issue2246_question_jump.py
index 1498b7871..dbf8cb597 100644
--- a/tests/test_issue2246_question_jump.py
+++ b/tests/test_issue2246_question_jump.py
@@ -36,6 +36,11 @@ def test_multi_segment_turn_jumps_to_first_assistant_segment():
 
 
 def test_question_jump_expands_windowed_history_and_highlights_question():
+    assert "function _messageVisibleIndexForRawIdx(rawIdx, visWithIdx)" in UI_JS
+    assert "function _messageVirtualScrollTopForVisibleIdx(visWithIdx, visibleIdx, container)" in UI_JS
+    assert "const visibleIdx=_messageVisibleIndexForRawIdx(questionRawIdx, visWithIdx);" in UI_JS
+    assert "container.scrollTop=_messageVirtualScrollTopForVisibleIdx(visWithIdx, visibleIdx, container);" in UI_JS
+    assert "_messageVirtualWindowKey='';" in UI_JS
     assert "_messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount())" in UI_JS
     assert "renderMessages({ preserveScroll:true })" in UI_JS
     assert "row.scrollIntoView({block:'center',behavior:'smooth'})" in UI_JS
diff --git a/tests/test_issue401.py b/tests/test_issue401.py
index d63477573..d82f7650e 100644
--- a/tests/test_issue401.py
+++ b/tests/test_issue401.py
@@ -72,14 +72,16 @@ def test_loadsession_uses_session_toolcalls_only_as_fallback():
 
 def test_rendermessages_treats_openai_toolcall_assistants_as_visible():
     """OpenAI assistant rows with empty content but tool_calls must stay anchorable."""
+    assert "function _messageIsRenderable(m)" in UI_JS
     assert "const hasTc=Array.isArray(m.tool_calls)&&m.tool_calls.length>0;" in UI_JS
-    assert "if(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)) return true;" in UI_JS
+    assert "hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)" in UI_JS
 
 
 def test_rendermessages_treats_partial_toolcall_assistants_as_visible():
     """Assistant rows carrying `_partial_tool_calls` must stay anchorable."""
+    assert "function _messageIsRenderable(m)" in UI_JS
     assert "const hasPartialTc=Array.isArray(m._partial_tool_calls)&&m._partial_tool_calls.length>0;" in UI_JS
-    assert "if(hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)) return true;" in UI_JS
+    assert "hasTc||hasTu||hasPartialTc||_messageHasReasoningPayload(m)||_assistantMessageHasVisibleContent(m)" in UI_JS
 
 
 def test_rendermessages_rebuilds_tool_cards_from_partial_tool_calls():
diff --git a/tests/test_issue500_message_list_virtualization.py b/tests/test_issue500_message_list_virtualization.py
new file mode 100644
index 000000000..3401f5cf6
--- /dev/null
+++ b/tests/test_issue500_message_list_virtualization.py
@@ -0,0 +1,364 @@
+"""Regression coverage for issue #500 transcript virtualization."""
+import json
+import shutil
+import subprocess
+import tempfile
+from pathlib import Path
+
+import pytest
+
+REPO_ROOT = Path(__file__).parent.parent.resolve()
+UI_JS_PATH = REPO_ROOT / "static" / "ui.js"
+NODE = shutil.which("node")
+
+pytestmark = pytest.mark.skipif(NODE is None, reason="node not on PATH")
+
+
+def _run_node(source: str) -> str:
+    with tempfile.NamedTemporaryFile(
+        "w", suffix=".cjs", encoding="utf-8", dir=REPO_ROOT, delete=False
+    ) as script:
+        script.write(source)
+        script_path = Path(script.name)
+    try:
+        result = subprocess.run(
+            [NODE, str(script_path)],
+            cwd=str(REPO_ROOT),
+            capture_output=True,
+            text=True,
+            timeout=10,
+        )
+    finally:
+        script_path.unlink(missing_ok=True)
+    if result.returncode != 0:
+        raise RuntimeError(result.stderr)
+    return result.stdout.strip()
+
+
+def _extract_func_script(js: str) -> str:
+    return f"""
+const src = {js!r};
+function extractFunc(name) {{
+  const re = new RegExp('function\\\\s+' + name + '\\\\s*\\\\(');
+  const start = src.search(re);
+  if (start < 0) throw new Error(name + ' not found');
+  let i = src.indexOf('{{', start);
+  let depth = 1; i++;
+  while (depth > 0 && i < src.length) {{
+    if (src[i] === '{{') depth++;
+    else if (src[i] === '}}') depth--;
+    i++;
+  }}
+  return src.slice(start, i);
+}}
+"""
+
+
+def test_message_virtual_window_virtualizes_older_history_but_keeps_recent_tail():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+eval(extractFunc('_messageVirtualWindow'));
+const metrics = _messageVirtualWindow({
+  total: 240,
+  scrollTop: 120 * 70,
+  viewportHeight: 720,
+  heights: Array.from({length: 240}, (_, i) => i >= 190 ? 220 : 120),
+  defaultHeight: 120,
+  bufferPx: 240,
+  threshold: 80,
+  keepTailCount: 50,
+});
+console.log(JSON.stringify(metrics));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["virtualized"] is True
+    assert 60 <= metrics["start"] <= 75
+    assert metrics["end"] <= metrics["tailStart"] == 190
+    assert metrics["topPad"] > 0
+    assert metrics["bottomPad"] > 0
+
+
+def test_message_virtual_window_collapses_to_tail_only_near_bottom():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+eval(extractFunc('_messageVirtualWindow'));
+const metrics = _messageVirtualWindow({
+  total: 240,
+  scrollTop: 120 * 260,
+  viewportHeight: 720,
+  heights: Array.from({length: 240}, () => 120),
+  defaultHeight: 120,
+  bufferPx: 240,
+  threshold: 80,
+  keepTailCount: 50,
+});
+console.log(JSON.stringify(metrics));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["virtualized"] is True
+    assert metrics["start"] == metrics["tailStart"] == 190
+    assert metrics["end"] == metrics["tailStart"]
+    assert metrics["bottomPad"] == 0
+
+
+def test_render_messages_uses_virtual_window_and_spacer_measurement_path():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    render_start = js.index("function renderMessages(options)")
+    render_end = js.index("function _toolDisplayName", render_start)
+    render_body = js[render_start:render_end]
+
+    assert "_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount())" in render_body
+    assert "const renderVisibleIdxs=[" in render_body
+    assert "_messageVirtualSpacer(virtualWindow.topPad,'before')" in render_body
+    assert "_messageVirtualSpacer(virtualWindow.bottomPad,'after')" in render_body
+    assert "_updateMessageVirtualMeasurements(renderVisWithIdx, renderVisibleIdxs, virtualWindow);" in render_body
+    assert "const renderableRawIdxs=new Set(visWithIdx.map(e=>e.rawIdx));" in render_body
+    assert "if(virtualWindow.virtualized&&renderableRawIdxs.has(aIdx)&&!renderedRawIdxs.has(aIdx)) continue;" in render_body
+    assert "if(hasServerOlder){" in render_body
+    assert "_showEarlierRenderedMessages();" not in render_body
+    top_spacer_idx = render_body.index("_messageVirtualSpacer(virtualWindow.topPad,'before')")
+    indicator_idx = render_body.index("indicator.id='loadOlderIndicator';")
+    assert top_spacer_idx < indicator_idx, (
+        "renderMessages() must place the load-older affordance after the top "
+        "virtual spacer so it stays visible at the top of the rendered window."
+    )
+    gap_reset_idx = render_body.index("currentAssistantTurn=null;", render_body.index("_messageVirtualSpacer(virtualWindow.bottomPad,'after')") - 220)
+    gap_spacer_idx = render_body.index("_messageVirtualSpacer(virtualWindow.bottomPad,'after')")
+    assert gap_reset_idx < gap_spacer_idx, (
+        "renderMessages() must reset currentAssistantTurn before inserting the "
+        "virtual gap spacer so assistant bubbles do not merge across the head/tail boundary."
+    )
+
+
+def test_measurement_uses_one_primary_row_and_adjacent_activity_siblings_only():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+eval(extractFunc('_measureMessageVirtualRow'));
+const nextMessage = {
+  hasAttribute(name){ return name === 'data-msg-idx'; },
+  getBoundingClientRect(){ return {height: 999}; },
+  nextElementSibling: null,
+};
+const activityGroup = {
+  hasAttribute(){ return false; },
+  matches(selector){ return selector === '.tool-call-group,.tool-card-row,.agent-activity-thinking,.thinking-card-row'; },
+  getBoundingClientRect(){ return {height: 60}; },
+  nextElementSibling: {
+    hasAttribute(){ return false; },
+    matches(){ return false; },
+    getBoundingClientRect(){ return {height: 5000}; },
+    nextElementSibling: nextMessage,
+  },
+};
+const primary = {
+  classList: { contains(name){ return name === 'assistant-segment'; } },
+  getBoundingClientRect(){ return {height: 120}; },
+  nextElementSibling: activityGroup,
+};
+const inner = {
+  querySelector(selector){
+    if(selector === '[data-msg-idx="42"]') return primary;
+    return null;
+  },
+};
+console.log(JSON.stringify({
+  total: _measureMessageVirtualRow(inner, {rawIdx: 42}),
+}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["total"] == 180
+
+
+def test_virtual_keep_tail_count_stays_bounded_after_history_expands_render_window():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+const MESSAGE_RENDER_WINDOW_DEFAULT = 50;
+let _messageRenderWindowSize = 240;
+eval(extractFunc('_currentMessageRenderWindowSize'));
+eval(extractFunc('_messageVirtualKeepTailCount'));
+console.log(JSON.stringify({
+  renderWindowSize: _currentMessageRenderWindowSize(),
+  keepTailCount: _messageVirtualKeepTailCount(),
+}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["renderWindowSize"] == 240
+    assert metrics["keepTailCount"] == 50
+
+
+def test_virtual_prepended_height_delta_uses_prefix_cache_only_when_virtualized():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+let virtualized = true;
+let _messageVirtualHeightCache = [0, 220, 180, 120];
+let _messageVirtualEstimatedRowHeight = 140;
+function _getVisibleMessagesWithIdx(){
+  return [{rawIdx: 0}, {rawIdx: 1}, {rawIdx: 2}, {rawIdx: 3}];
+}
+function _messageVirtualKeepTailCount(){ return 2; }
+function _currentMessageVirtualWindow(){
+  return {virtualized};
+}
+eval(extractFunc('_messageVirtualPrependedHeightDelta'));
+const active = _messageVirtualPrependedHeightDelta(3);
+virtualized = false;
+const inactive = _messageVirtualPrependedHeightDelta(3);
+console.log(JSON.stringify({active, inactive}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["active"] == 540
+    assert metrics["inactive"] is None
+
+
+def test_virtual_question_jump_scroll_target_uses_visible_index_height_prefix():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+let _messageVirtualHeightCache = [100, 120, 80, 140];
+let _messageVirtualHeightCacheEntries = [];
+let _messageVirtualHeightCacheLen = 4;
+let _messageVirtualHeightCacheSrc = null;
+let _messageVirtualEstimatedRowHeight = 110;
+let _messageVirtualWindowKey = 'old';
+let S = {messages: [{}, {}, {}, {}]};
+function _messageIsRenderable(){ return true; }
+eval(extractFunc('_messageVirtualHeightEntryMatches'));
+eval(extractFunc('_syncMessageVirtualHeightCache'));
+eval(extractFunc('_messageVisibleIndexForRawIdx'));
+eval(extractFunc('_messageVirtualScrollTopForVisibleIdx'));
+const visWithIdx = [
+  {rawIdx: 10, m: S.messages[0]},
+  {rawIdx: 12, m: S.messages[1]},
+  {rawIdx: 14, m: S.messages[2]},
+  {rawIdx: 16, m: S.messages[3]},
+];
+_messageVirtualHeightCacheEntries = visWithIdx;
+_messageVirtualHeightCacheSrc = S.messages;
+const visibleIdx = _messageVisibleIndexForRawIdx(14, visWithIdx);
+const scrollTop = _messageVirtualScrollTopForVisibleIdx(visWithIdx, visibleIdx, {clientHeight: 200});
+console.log(JSON.stringify({visibleIdx, scrollTop}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["visibleIdx"] == 2
+    assert metrics["scrollTop"] == 150
+
+
+def test_height_cache_preserves_measured_prefix_across_append_only_growth():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+const MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT = 140;
+let _messageVirtualHeightCache = [180, 220];
+let _messageVirtualHeightCacheEntries = [];
+let _messageVirtualHeightCacheLen = 2;
+let _messageVirtualHeightCacheSrc = null;
+let _messageVirtualEstimatedRowHeight = 200;
+let _messageVirtualWindowKey = 'stale-key';
+function _clearMessageVirtualHeightCache() {
+  _messageVirtualHeightCache = [];
+  _messageVirtualHeightCacheEntries = [];
+  _messageVirtualHeightCacheLen = 0;
+  _messageVirtualHeightCacheSrc = null;
+  _messageVirtualEstimatedRowHeight = MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT;
+  _messageVirtualWindowKey = '';
+}
+eval(extractFunc('_messageVirtualHeightEntryMatches'));
+eval(extractFunc('_messageVirtualHeightPrefixEntryMatches'));
+eval(extractFunc('_syncMessageVirtualHeightCache'));
+const first = {id: 'first'};
+const second = {id: 'second'};
+let S = {messages: [first, second]};
+_messageVirtualHeightCacheEntries = [
+  {rawIdx: 0, m: first},
+  {rawIdx: 1, m: second},
+];
+_messageVirtualHeightCacheSrc = S.messages;
+S = {messages: [first, second, {id: 'third'}]};
+_syncMessageVirtualHeightCache([
+  {rawIdx: 0, m: first},
+  {rawIdx: 1, m: second},
+  {rawIdx: 2, m: S.messages[2]},
+]);
+console.log(JSON.stringify({
+  cache: _messageVirtualHeightCache,
+  estimated: _messageVirtualEstimatedRowHeight,
+  windowKey: _messageVirtualWindowKey,
+}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["cache"][:2] == [180, 220]
+    assert len(metrics["cache"]) == 3
+    assert metrics["estimated"] == 200
+    assert metrics["windowKey"] == ""
+
+
+def test_height_cache_preserves_measured_suffix_across_prepended_history():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+const MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT = 140;
+let _messageVirtualHeightCache = [180, 220];
+let _messageVirtualHeightCacheEntries = [];
+let _messageVirtualHeightCacheLen = 2;
+let _messageVirtualHeightCacheSrc = null;
+let _messageVirtualEstimatedRowHeight = 200;
+let _messageVirtualWindowKey = 'stale-key';
+function _clearMessageVirtualHeightCache() {
+  _messageVirtualHeightCache = [];
+  _messageVirtualHeightCacheEntries = [];
+  _messageVirtualHeightCacheLen = 0;
+  _messageVirtualHeightCacheSrc = null;
+  _messageVirtualEstimatedRowHeight = MESSAGE_VIRTUAL_DEFAULT_ROW_HEIGHT;
+  _messageVirtualWindowKey = '';
+}
+eval(extractFunc('_messageVirtualHeightEntryMatches'));
+eval(extractFunc('_messageVirtualHeightPrefixEntryMatches'));
+eval(extractFunc('_syncMessageVirtualHeightCache'));
+const first = {id: 'first'};
+const second = {id: 'second'};
+let S = {messages: [first, second]};
+_messageVirtualHeightCacheEntries = [
+  {rawIdx: 0, m: first},
+  {rawIdx: 1, m: second},
+];
+_messageVirtualHeightCacheSrc = S.messages;
+const olderA = {id: 'older-a'};
+const olderB = {id: 'older-b'};
+S = {messages: [olderA, olderB, first, second]};
+_syncMessageVirtualHeightCache([
+  {rawIdx: 0, m: olderA},
+  {rawIdx: 1, m: olderB},
+  {rawIdx: 2, m: first},
+  {rawIdx: 3, m: second},
+]);
+console.log(JSON.stringify({
+  cache: _messageVirtualHeightCache,
+  estimated: _messageVirtualEstimatedRowHeight,
+  windowKey: _messageVirtualWindowKey,
+}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["cache"][2:] == [180, 220]
+    assert len(metrics["cache"]) == 4
+    assert metrics["estimated"] == 200
+    assert metrics["windowKey"] == ""
+
+
+def test_measurement_refresh_budget_is_keyed_to_window_shape_not_pad_height():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    source = _extract_func_script(js) + """
+eval(extractFunc('_messageVirtualMeasurementCycleKeyFor'));
+console.log(JSON.stringify({
+  a: _messageVirtualMeasurementCycleKeyFor({virtualized: true, start: 10, end: 20, topPad: 1000, bottomPad: 2000, tailStart: 190}),
+  b: _messageVirtualMeasurementCycleKeyFor({virtualized: true, start: 10, end: 20, topPad: 1001, bottomPad: 1999, tailStart: 190}),
+}));
+"""
+    metrics = json.loads(_run_node(source))
+    assert metrics["a"] == metrics["b"]
+
+
+def test_tool_rows_do_not_carry_message_measurement_hook():
+    js = UI_JS_PATH.read_text(encoding="utf-8")
+    build_start = js.index("function buildToolCard(tc){")
+    build_end = js.index("function _colorDiffLines", build_start)
+    build_body = js[build_start:build_end]
+
+    assert "row.dataset.msgIdx" not in build_body
+    assert "querySelectorAll(`[data-msg-idx=" not in js
diff --git a/tests/test_issue734_message_windowing.py b/tests/test_issue734_message_windowing.py
index 0352f0b66..adf7df9e6 100644
--- a/tests/test_issue734_message_windowing.py
+++ b/tests/test_issue734_message_windowing.py
@@ -1,26 +1,30 @@
 from pathlib import Path
 
 REPO = Path(__file__).resolve().parents[1]
-UI_JS = (REPO / "static" / "ui.js").read_text()
-CSS = (REPO / "static" / "style.css").read_text()
+UI_JS = (REPO / "static" / "ui.js").read_text(encoding="utf-8")
+SESSIONS_JS = (REPO / "static" / "sessions.js").read_text(encoding="utf-8")
+CSS = (REPO / "static" / "style.css").read_text(encoding="utf-8")
 
 
-def test_message_windowing_caps_initial_dom_to_recent_messages():
-    assert "const MESSAGE_RENDER_WINDOW_DEFAULT=50" in UI_JS
-    assert "_messageRenderWindowSize=MESSAGE_RENDER_WINDOW_DEFAULT" in UI_JS
-    assert "const windowStart=Math.max(0, visWithIdx.length-renderWindowSize)" in UI_JS
-    assert "const renderVisWithIdx=visWithIdx.slice(windowStart)" in UI_JS
-    assert "for(let vi=0;vi<renderVisWithIdx.length;vi++)" in UI_JS
-    assert "Load earlier messages (${hiddenBeforeCount} hidden)" in UI_JS
+def test_message_virtualization_switches_render_messages_to_scroll_driven_window():
+    assert "function _messageVirtualWindow(opts)" in UI_JS
+    assert "function _messageVirtualKeepTailCount()" in UI_JS
+    assert "const virtualWindow=_currentMessageVirtualWindow(visWithIdx,_messageVirtualKeepTailCount())" in UI_JS
+    assert "const renderHeadVisWithIdx=visWithIdx.slice(windowStart, windowEnd)" in UI_JS
+    assert "const renderTailStart=virtualWindow.virtualized?Math.max(windowEnd, virtualWindow.tailStart):windowEnd" in UI_JS
+    assert "const renderTailVisWithIdx=virtualWindow.virtualized&&renderTailStart<visWithIdx.length" in UI_JS
+    assert "const renderVisWithIdx=renderHeadVisWithIdx.concat(renderTailVisWithIdx)" in UI_JS
+    assert "if(virtualWindow.virtualized&&virtualWindow.bottomPad>0&&vi===headRenderCount)" in UI_JS
 
 
-def test_load_earlier_expands_local_window_before_server_pagination_and_preserves_scroll():
-    assert "function _showEarlierRenderedMessages()" in UI_JS
-    assert "prevScrollH=container?container.scrollHeight:0" in UI_JS
-    assert "prevScrollTop=container?container.scrollTop:0" in UI_JS
-    assert "container.scrollTop=prevScrollTop+(newScrollH-prevScrollH)" in UI_JS
-    assert "if(_messageHiddenBeforeCount()>0) _showEarlierRenderedMessages();" in UI_JS
-    assert "else if(typeof _loadOlderMessages==='function') _loadOlderMessages();" in UI_JS
+def test_load_earlier_only_pages_server_history_and_preserves_scroll():
+    assert "function _wireMessageWindowLoadEarlierButton()" in UI_JS
+    assert "if(typeof _loadOlderMessages==='function') _loadOlderMessages();" in UI_JS
+    assert "if(hasServerOlder){" in UI_JS
+    assert "if(virtualWindow.virtualized&&virtualWindow.topPad>0)" in UI_JS
+    assert "_messageRenderWindowSize=_currentMessageRenderWindowSize()+Math.max(addedRenderable, MESSAGE_RENDER_WINDOW_DEFAULT);" in SESSIONS_JS
+    assert "renderMessages({ preserveScroll: true });" in SESSIONS_JS
+    assert "_scheduleMessageVirtualizedRender();" in UI_JS
 
 
 def test_windowed_render_keeps_streaming_and_tool_activity_anchored_to_rendered_messages():
@@ -32,15 +36,26 @@ def test_windowed_render_keeps_streaming_and_tool_activity_anchored_to_rendered_
 
 
 def test_window_state_participates_in_cache_and_cached_button_is_rewired():
-    assert "cached.renderWindowSize===renderWindowSize" in UI_JS
+    assert "cached.renderWindowKey===renderWindowKey" in UI_JS
     assert "cached.signature===renderSignature" in UI_JS
-    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowSize,signature:renderSignature})" in UI_JS
+    assert "_sessionHtmlCache.set(sid,{html:_html,msgCount,renderWindowKey,signature:renderSignature})" in UI_JS
+    assert "_messageVirtualWindowKey=renderWindowKey" in UI_JS
     assert "function _wireMessageWindowLoadEarlierButton()" in UI_JS
     assert "_wireMessageWindowLoadEarlierButton();" in UI_JS
     assert UI_JS.count("_wireMessageWindowLoadEarlierButton();") >= 2
 
 
-def test_load_earlier_affordance_has_button_styling_hook():
+def test_virtualization_affordances_have_styling_hooks():
     assert "message-window-load-earlier" in UI_JS
     assert ".message-window-load-earlier" in CSS
+    assert ".message-virtual-spacer" in CSS
     assert "border-radius:999px" in CSS
+
+
+def test_measurement_rerenders_are_bounded_per_virtual_window_cycle():
+    assert "const MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS=2;" in UI_JS
+    assert "function _messageVirtualMeasurementCycleKeyFor(windowMetrics)" in UI_JS
+    assert "function _scheduleMessageVirtualMeasurementRefresh(windowMetrics)" in UI_JS
+    assert "if(_messageVirtualMeasurementRetryCount>=MESSAGE_VIRTUAL_MEASUREMENT_MAX_RERENDERS) return;" in UI_JS
+    assert "_scheduleMessageVirtualMeasurementRefresh(virtualWindow);" in UI_JS
+    assert "_markMessageVirtualMeasurementsSettled(virtualWindow);" in UI_JS
diff --git a/tests/test_live_stream_ux.py b/tests/test_live_stream_ux.py
index 2f03f34bf..abbf4e77c 100644
--- a/tests/test_live_stream_ux.py
+++ b/tests/test_live_stream_ux.py
@@ -27,7 +27,8 @@ def test_ui_rejects_recovery_control_as_visible_assistant_content():
     assert "function _isRecoveryControlMessageText" in UI_JS
     assert "function _assistantMessageHasVisibleContent" in UI_JS
     assert "if(_isRecoveryControlMessage(m)) return false;" in UI_JS
-    assert "if(_isRecoveryControlMessage(m)){ri++;continue;}" in UI_JS
+    assert "function _messageIsRenderable(m)" in UI_JS
+    assert "if(_isRecoveryControlMessage(m)) return false;" in UI_JS[UI_JS.index("function _messageIsRenderable(m)"):]
     assert "_assistantMessageHasVisibleContent(m)" in UI_JS
 
 
diff --git a/tests/test_older_history_viewport_preservation.py b/tests/test_older_history_viewport_preservation.py
index d34d50860..a9cf50143 100644
--- a/tests/test_older_history_viewport_preservation.py
+++ b/tests/test_older_history_viewport_preservation.py
@@ -29,6 +29,7 @@ def test_loading_older_messages_expands_render_window_before_rendering():
         "scroll-to-top paging must expand the DOM render window before renderMessages(); "
         "otherwise fetched older messages stay hidden and only the hidden counter changes"
     )
+    assert "if(typeof _messageIsRenderable==='function') return _messageIsRenderable(m);" in body
     assert "Math.max(addedRenderable, MESSAGE_RENDER_WINDOW_DEFAULT)" in body
 
 
@@ -36,14 +37,22 @@ def test_loading_older_messages_preserves_viewport_without_bottom_snap():
     body = _function_body(SESSIONS_JS, "async function _loadOlderMessages")
 
     assert "renderMessages({ preserveScroll: true });" in body
-    assert "const oldTop = container.scrollTop" in body
-    assert "const addedHeight = Math.max(0, newScrollH - prevScrollH)" in body
+    assert "const viewportAnchor = (container && typeof _captureMessageViewportAnchor === 'function')" in body
+    assert "_captureMessageViewportAnchor()" in body
+    assert "_restoreMessageViewportAnchor(viewportAnchor, olderMsgs.length)" in body
+    assert "const restoredViaAnchor = (viewportAnchor && typeof _restoreMessageViewportAnchor === 'function')" in body
+    assert "if (!restoredViaAnchor) {" in body
+    assert "const virtualAddedHeight = (typeof _messageVirtualPrependedHeightDelta === 'function')" in body
+    assert "_messageVirtualPrependedHeightDelta(addedRenderable)" in body
+    assert "const addedHeight = Number.isFinite(virtualAddedHeight)" in body
     assert "container.scrollTop = oldTop + addedHeight" in body
     assert "container.scrollTop = newScrollH - prevScrollH" not in body
 
-    restore_idx = body.index("container.scrollTop = oldTop + addedHeight")
+    restore_idx = body.index("_restoreMessageViewportAnchor(viewportAnchor, olderMsgs.length)")
+    virtual_idx = body.index("_messageVirtualPrependedHeightDelta(addedRenderable)")
+    scroll_delta_idx = body.index("Math.max(0, newScrollH - prevScrollH)")
     unpin_idx = body.rindex("_scrollPinned = false")
-    assert restore_idx < unpin_idx
+    assert restore_idx < virtual_idx < scroll_delta_idx < unpin_idx
 
 
 def test_loading_older_messages_marks_scroll_programmatic_while_anchoring():
@@ -53,3 +62,14 @@ def test_loading_older_messages_marks_scroll_programmatic_while_anchoring():
     restore_idx = body.index("container.scrollTop = oldTop + addedHeight")
     clear_idx = body.index("requestAnimationFrame(()=>{ _programmaticScroll = false; })")
     assert set_idx < restore_idx < clear_idx
+
+
+def test_loading_older_messages_captures_anchor_before_replacing_messages():
+    body = _function_body(SESSIONS_JS, "async function _loadOlderMessages")
+
+    anchor_idx = body.index("const viewportAnchor = (container && typeof _captureMessageViewportAnchor === 'function')")
+    replace_idx = body.index("S.messages = nextMessages")
+    render_idx = body.index("renderMessages({ preserveScroll: true });")
+    restore_idx = body.index("_restoreMessageViewportAnchor(viewportAnchor, olderMsgs.length)")
+
+    assert anchor_idx < replace_idx < render_idx < restore_idx
diff --git a/tests/test_session_jump_buttons.py b/tests/test_session_jump_buttons.py
index fee564760..8229f170f 100644
--- a/tests/test_session_jump_buttons.py
+++ b/tests/test_session_jump_buttons.py
@@ -50,6 +50,11 @@ def test_jump_to_session_start_button_loads_full_history_and_scrolls_top():
 
     assert "_ensureAllMessagesLoaded" in jump
     assert "_messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount())" in jump
+    expand_idx = jump.index("_messageRenderWindowSize=Math.max(_currentMessageRenderWindowSize(),_messageRenderableMessageCount())")
+    top_idx = jump.index("container.scrollTop=0")
+    render_idx = jump.index("renderMessages({ preserveScroll:true })")
+    assert expand_idx < top_idx < render_idx
+    assert "_messageVirtualWindowKey=''" in jump
     assert "renderMessages({ preserveScroll:true })" in jump
     assert "container.scrollTop=0" in jump
     assert "btn.style.display=(hasSession&&canRevealStart&&awayFromStart)?'flex':'none'" in update
diff --git a/tests/test_update_banner_fixes.py b/tests/test_update_banner_fixes.py
index 1cb2b80a5..b7ec83667 100644
--- a/tests/test_update_banner_fixes.py
+++ b/tests/test_update_banner_fixes.py
@@ -409,6 +409,8 @@ class TestScheduleRestart:
         import os as _os
         original_execv = _os.execv
 
+        monkeypatch.setattr(sys, 'platform', 'linux')
+        monkeypatch.setattr(upd, '_wait_until_restart_safe', lambda *a, **k: {'restart_blocked': False})
         monkeypatch.setattr(_os, 'execv', fake_execv)
 
         start = time.monotonic()
@@ -439,6 +441,8 @@ class TestScheduleRestart:
             events.append(("execv", exe))
 
         # Override the autouse no-op stub with a recording spy.
+        monkeypatch.setattr(sys, 'platform', 'linux')
+        monkeypatch.setattr(upd, '_wait_until_restart_safe', lambda *a, **k: {'restart_blocked': False})
         monkeypatch.setattr(upd, "_purge_agent_pycache", spy_purge)
         monkeypatch.setattr(os, "execv", fake_execv)
 
@@ -1523,6 +1527,8 @@ class TestSequentialUpdateRestartCoordination:
             execv_time.append(_t.monotonic())
             execv_called.set()
 
+        monkeypatch.setattr(sys, 'platform', 'linux')
+        monkeypatch.setattr(upd, '_wait_until_restart_safe', lambda *a, **k: {'restart_blocked': False})
         monkeypatch.setattr(os, 'execv', fake_execv)
 
         # Hold _apply_lock from another thread (simulating an in-flight
@@ -1570,6 +1576,8 @@ class TestSequentialUpdateRestartCoordination:
         execv_called = []
         def fake_execv(exe, args):
             execv_called.append(True)
+        monkeypatch.setattr(sys, 'platform', 'linux')
+        monkeypatch.setattr(upd, '_wait_until_restart_safe', lambda *a, **k: {'restart_blocked': False})
         monkeypatch.setattr(os, 'execv', fake_execv)
 
         upd._schedule_restart(delay=0.05)