diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0feebddb..341a2d7f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,12 @@
 
 ## [Unreleased]
 
+## [v0.51.592] — 2026-06-22 — Release UY (skip gateway poll when hidden/streaming + smd re-init)
+
+### Fixed
+
+- **The sidebar's gateway-poll fallback no longer burns CPU re-rendering while you're streaming or the tab is hidden, and long streamed responses render more efficiently.** The 5s gateway-poll fallback now skips its `renderSessionList` pass when the tab is hidden or a turn is actively streaming (it catches up immediately on tab refocus via a one-time `visibilitychange` listener, so no gateway updates are dropped). Separately, when the streaming markdown parser was torn down by a prior segment but `window.smd` is still available, the live renderer now recreates the parser on the cleared element instead of falling back to repeated full-`innerHTML` rebuilds — avoiding O(n²) DOM churn on long responses. Thanks @akrhin. (#4730)
+
 ## [v0.51.591] — 2026-06-22 — Release UX (stop transcript jumping to first message on completion)
 
 ### Fixed
diff --git a/static/messages.js b/static/messages.js
index cf15b25b..2c1a02d0 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -3352,6 +3352,18 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
       : _stripXmlToolCalls(assistantText.slice(segmentStart));
     if(_smdParser){
       _smdWrite(displayText);
+    } else if(window.smd){
+      // Parser was nulled out (e.g. by a prior segment end) but smd is
+      // available — recreate it on the existing element. Uses the non-fade
+      // renderer to match standard rendering, avoiding O(n²) innerHTML
+      // churn on long responses (#4704). Clear any content the renderMd()
+      // fallback already wrote first: _smdNewParser resets _smdWrittenText to
+      // '' but does NOT clear the element, so a following _smdWrite(displayText)
+      // would append the full accumulated segment ON TOP of the existing
+      // fallback render and duplicate the live text.
+      assistantBody.innerHTML='';
+      _smdNewParser(assistantBody, false);
+      if(_smdParser) _smdWrite(displayText);
     } else if(renderMd){
       assistantBody.innerHTML=renderMd(displayText);
     } else {
diff --git a/static/sessions.js b/static/sessions.js
index 062fd639..1867768e 100644
--- a/static/sessions.js
+++ b/static/sessions.js
@@ -4101,7 +4101,23 @@ if(typeof window!=='undefined') window.refreshSessionList = refreshSessionList;
 function startGatewayPollFallback(ms){
   const intervalMs = Math.max(5000, Number(ms) || _gatewayFallbackPollMs);
   if(_gatewayPollTimer) clearInterval(_gatewayPollTimer);
-  _gatewayPollTimer = setInterval(() => { renderSessionList({deferWhileInteracting:true}); }, intervalMs);
+  _gatewayPollTimer = setInterval(() => {
+    // Skip poll when tab is hidden or a stream is active — saves CPU
+    // and avoids redundant DOM renders during active streaming (#4704).
+    if(typeof document !== 'undefined' && document.hidden) return;
+    if(typeof S !== 'undefined' && (S.busy || S.activeStreamId)) return;
+    renderSessionList({deferWhileInteracting:true});
+  }, intervalMs);
+  // Visibility catch-up: refresh immediately when tab re-gains focus,
+  // so no gateway updates are dropped during hidden-skip periods.
+  if(typeof document !== 'undefined' && !document._hermesGatewayPollVisibilityHook){
+    document.addEventListener('visibilitychange', () => {
+      if(!document.hidden && typeof renderSessionList === 'function'){
+        void renderSessionList({deferWhileInteracting:false});
+      }
+    });
+    document._hermesGatewayPollVisibilityHook = true;
+  }
 }
 
 function stopGatewayPollFallback(){