fix(tui): keep DECSTBM scroll region off bottom row (#26683)

Avoid shifting the terminal's last visible row in the alt-screen DECSTBM fast path, which can leave transient scroll bleed/discoloration artifacts around the status lane until a repaint. Add regression tests to preserve the fast path when safe and skip it when the hint touches the bottom row.

ui-tui/packages/hermes-ink/src/ink/log-update.test.ts

@@ -42,6 +42,8 @@ const stdoutOnly = (diff: ReturnType<LogUpdate['render']>) =>
     .map(p => (p as { type: 'stdout'; content: string }).content)
     .join('')
 
+const hasDecstbm = (text: string) => /\x1b\[\d+;\d+r/.test(text)
+
 describe('LogUpdate.render diff contract', () => {
   it('emits only changed cells when most rows match', () => {
     const w = 20
@@ -154,4 +156,44 @@ describe('LogUpdate.render diff contract', () => {
     expect(diff.some(p => p.type === 'clearTerminal')).toBe(true)
     expect(stdoutOnly(diff)).toContain('timer2s')
   })
+
+  it('keeps DECSTBM fast-path when scroll region stays above bottom row', () => {
+    const w = 12
+    const h = 6
+    const prev = mkScreen(w, h)
+    const next = mkScreen(w, h)
+
+    paint(prev, 1, 'row one')
+    paint(next, 1, 'row one')
+
+    const prevFrame = mkFrame(prev, w, h)
+    const nextFrame: Frame = {
+      ...mkFrame(next, w, h),
+      scrollHint: { top: 1, bottom: 4, delta: 1 }
+    }
+    const log = new LogUpdate({ isTTY: true, stylePool })
+    const diff = log.render(prevFrame, nextFrame, true, true)
+
+    expect(hasDecstbm(stdoutOnly(diff))).toBe(true)
+  })
+
+  it('skips DECSTBM when scroll region touches the bottom row', () => {
+    const w = 12
+    const h = 6
+    const prev = mkScreen(w, h)
+    const next = mkScreen(w, h)
+
+    paint(prev, 1, 'row one')
+    paint(next, 1, 'row one')
+
+    const prevFrame = mkFrame(prev, w, h)
+    const nextFrame: Frame = {
+      ...mkFrame(next, w, h),
+      scrollHint: { top: 1, bottom: 5, delta: 1 }
+    }
+    const log = new LogUpdate({ isTTY: true, stylePool })
+    const diff = log.render(prevFrame, nextFrame, true, true)
+
+    expect(hasDecstbm(stdoutOnly(diff))).toBe(false)
+  })
 })

ui-tui/packages/hermes-ink/src/ink/log-update.ts

@@ -175,7 +175,10 @@ export class LogUpdate {
     if (altScreen && next.scrollHint && decstbmSafe) {
       const { top, bottom, delta } = next.scrollHint
 
-      if (top >= 0 && bottom < prev.screen.height && bottom < next.screen.height) {
+      // Keep DECSTBM away from the terminal's last visible row. In alt-screen
+      // layouts we reserve that lane for status/cursor parking, and scrolling
+      // it can leave transient ghosting/bleed artifacts until a later repaint.
+      if (top >= 0 && bottom < prev.screen.height - 1 && bottom < next.screen.height - 1) {
         shiftRows(prev.screen, top, bottom, delta)
         scrollPatch = [
           {