fix(tui): stabilize live progress rendering

ui-tui/src/__tests__/turnStore.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  freezeTurnRendering,
+  getRenderableTurnState,
+  patchTurnState,
+  resetTurnState,
+  unfreezeTurnRendering
+} from '../app/turnStore.js'
+
+describe('turn render freezing', () => {
+  it('holds the render snapshot stable while live turn state keeps changing', () => {
+    resetTurnState()
+    patchTurnState({ streaming: 'before scroll' })
+    freezeTurnRendering()
+
+    patchTurnState({ reasoning: 'new thinking', streaming: 'new streamed text' })
+
+    expect(getRenderableTurnState().streaming).toBe('before scroll')
+    expect(getRenderableTurnState().reasoning).toBe('')
+
+    unfreezeTurnRendering()
+
+    expect(getRenderableTurnState().streaming).toBe('new streamed text')
+    expect(getRenderableTurnState().reasoning).toBe('new thinking')
+  })
+})