fix(tui): stabilize sticky prompt tracking

Keep the latest prompt sticky while the viewport is in live assistant output beyond history, and clear stale sticky state at the real bottom using fresh scroll height.

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

@@ -28,4 +28,31 @@ describe('stickyPromptFromViewport', () => {
 
     expect(stickyPromptFromViewport(messages, offsets, 16, 20, false)).toBe('current prompt')
   })
+
+  it('shows the last prompt once the viewport starts after the history tail', () => {
+    const messages = [
+      { role: 'user' as const, text: 'current prompt' },
+      { role: 'assistant' as const, text: 'completed answer' }
+    ]
+
+    expect(stickyPromptFromViewport(messages, [0, 2, 5], 8, 14, false)).toBe('current prompt')
+  })
+
+  it('shows a prompt as soon as its full row is above the viewport', () => {
+    const messages = [
+      { role: 'user' as const, text: 'current prompt' },
+      { role: 'assistant' as const, text: 'current answer' }
+    ]
+
+    expect(stickyPromptFromViewport(messages, [0, 2, 10], 2, 8, false)).toBe('current prompt')
+  })
+
+  it('hides the sticky prompt at the bottom', () => {
+    const messages = [
+      { role: 'user' as const, text: 'current prompt' },
+      { role: 'assistant' as const, text: 'current answer' }
+    ]
+
+    expect(stickyPromptFromViewport(messages, [0, 2, 10], 8, 10, true)).toBe('')
+  })
 })

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

@@ -35,4 +35,20 @@ describe('viewportStore', () => {
     })
     expect(viewportSnapshotKey(snap)).toBe('0:16:5:40:3')
   })
+
+  it('uses fresh scroll height to clear stale non-bottom state', () => {
+    const handle = {
+      getFreshScrollHeight: () => 20,
+      getPendingDelta: () => 0,
+      getScrollHeight: () => 40,
+      getScrollTop: () => 15,
+      getViewportHeight: () => 5,
+      isSticky: () => false
+    }
+
+    const snap = getViewportSnapshot(handle as any)
+
+    expect(snap.atBottom).toBe(true)
+    expect(snap.scrollHeight).toBe(20)
+  })
 })

ui-tui/src/domain/viewport.ts

@@ -26,21 +26,25 @@ export const stickyPromptFromViewport = (
     return ''
   }
 
-  const first = Math.max(0, Math.min(messages.length - 1, upperBound(offsets, top) - 1))
-  const last = Math.max(first, Math.min(messages.length - 1, upperBound(offsets, bottom) - 1))
+  const first = Math.max(0, upperBound(offsets, top) - 1)
+  const last = Math.max(first, upperBound(offsets, bottom) - 1)
+  const visibleStart = Math.min(messages.length, first)
+  const visibleEnd = Math.min(messages.length - 1, last)
 
-  for (let i = first; i <= last; i++) {
+  for (let i = visibleStart; i <= visibleEnd; i++) {
     if (messages[i]?.role === 'user') {
       return ''
     }
   }
 
-  for (let i = first - 1; i >= 0; i--) {
+  for (let i = Math.min(messages.length - 1, visibleStart - 1); i >= 0; i--) {
     if (messages[i]?.role !== 'user') {
       continue
     }
 
-    return (offsets[i] ?? 0) + 1 < top ? userDisplay(messages[i]!.text.trim()).replace(/\s+/g, ' ').trim() : ''
+    return (offsets[i + 1] ?? (offsets[i] ?? 0) + 1) <= top
+      ? userDisplay(messages[i]!.text.trim()).replace(/\s+/g, ' ').trim()
+      : ''
   }
 
   return ''

ui-tui/src/lib/viewportStore.ts

@@ -28,11 +28,18 @@ export function getViewportSnapshot(s?: ScrollBoxHandle | null): ViewportSnapsho
   const pending = s.getPendingDelta()
   const top = Math.max(0, s.getScrollTop() + pending)
   const viewportHeight = Math.max(0, s.getViewportHeight())
-  const scrollHeight = Math.max(viewportHeight, s.getScrollHeight())
+  const cachedScrollHeight = Math.max(viewportHeight, s.getScrollHeight())
+  let scrollHeight = cachedScrollHeight
   const bottom = top + viewportHeight
+  let atBottom = s.isSticky() || bottom >= scrollHeight - 2
+
+  if (!atBottom) {
+    scrollHeight = Math.max(viewportHeight, s.getFreshScrollHeight?.() ?? cachedScrollHeight)
+    atBottom = s.isSticky() || bottom >= scrollHeight - 2
+  }
 
   return {
-    atBottom: s.isSticky() || bottom >= scrollHeight - 2,
+    atBottom,
     bottom,
     pending,
     scrollHeight,

ui-tui/src/types/hermes-ink.d.ts

@@ -83,6 +83,7 @@ declare module '@hermes/ink' {
     readonly getScrollTop: () => number
     readonly getPendingDelta: () => number
     readonly getScrollHeight: () => number
+    readonly getFreshScrollHeight: () => number
     readonly getViewportHeight: () => number
     readonly getViewportTop: () => number
     readonly getLastManualScrollAt: () => number