From c8ff70fe03f5c0fb5726392bed9586544b1d8b15 Mon Sep 17 00:00:00 2001
From: Brooklyn Nicholson <brooklyn.bb.nicholson@gmail.com>
Date: Thu, 23 Apr 2026 13:16:18 -0500
Subject: [PATCH] perf(ui-tui): freeze offscreen live tail during scroll

When the viewport is away from the bottom, keep the last visible progress snapshot instead of rebuilding the streaming/thinking subtree on every turn-store update. This cuts scroll-time churn while preserving live updates near the tail and on turn completion.
---
 ui-tui/src/app/useMainApp.ts | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/ui-tui/src/app/useMainApp.ts b/ui-tui/src/app/useMainApp.ts
index 39c4b534c..fdfdd8d54 100644
--- a/ui-tui/src/app/useMainApp.ts
+++ b/ui-tui/src/app/useMainApp.ts
@@ -22,7 +22,7 @@ import type { Msg, PanelSection, SlashCatalog } from '../types.js'
 
 import { createGatewayEventHandler } from './createGatewayEventHandler.js'
 import { createSlashHandler } from './createSlashHandler.js'
-import { type GatewayRpc, type TranscriptRow } from './interfaces.js'
+import { type AppLayoutProgressProps, type GatewayRpc, type TranscriptRow } from './interfaces.js'
 import { $overlayState, patchOverlayState } from './overlayStore.js'
 import { turnController } from './turnController.js'
 import { $turnState, patchTurnState } from './turnStore.js'
@@ -658,11 +658,36 @@ export function useMainApp(gw: GatewayClient) {
     [cols, composerActions, composerState, empty, pagerPageSize, submit]
   )
 
-  const appProgress = useMemo(
+  const liveTailVisible = (() => {
+    const s = scrollRef.current
+
+    if (!s) {
+      return true
+    }
+
+    const top = Math.max(0, s.getScrollTop() + s.getPendingDelta())
+    const vp = Math.max(0, s.getViewportHeight())
+    const total = Math.max(vp, s.getScrollHeight())
+
+    return top + vp >= total - 3
+  })()
+
+  const liveProgress = useMemo<AppLayoutProgressProps>(
     () => ({ ...turn, showProgressArea, showStreamingArea: Boolean(turn.streaming) }),
     [turn, showProgressArea]
   )
 
+  const frozenProgressRef = useRef(liveProgress)
+
+  // When the live tail is offscreen, freeze its snapshot so scroll work doesn't
+  // keep rebuilding the streaming/thinking subtree the user can't see. Thaw as
+  // soon as the viewport comes back near the bottom or the turn finishes.
+  if (liveTailVisible || !ui.busy) {
+    frozenProgressRef.current = liveProgress
+  }
+
+  const appProgress = liveTailVisible || !ui.busy ? liveProgress : frozenProgressRef.current
+
   const cwd = ui.info?.cwd || process.env.HERMES_CWD || process.cwd()
   const gitBranch = useGitBranch(cwd)