perf(tui): stabilize long-session scrolling

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

@@ -1,6 +1,7 @@
 import { describe, expect, it } from 'vitest'
 
-import { cursorLayout, offsetFromPosition } from '../components/textInput.js'
+import { offsetFromPosition } from '../components/textInput.js'
+import { cursorLayout, inputVisualHeight, stableComposerColumns } from '../lib/inputMetrics.js'
 
 describe('cursorLayout — char-wrap parity with wrap-ansi', () => {
   it('places cursor mid-line at its column', () => {
@@ -35,6 +36,18 @@ describe('cursorLayout — char-wrap parity with wrap-ansi', () => {
   })
 })
 
+describe('input metrics helpers', () => {
+  it('computes visual height from the wrapped cursor line', () => {
+    expect(inputVisualHeight('abcdefgh', 8)).toBe(2)
+    expect(inputVisualHeight('one\ntwo', 40)).toBe(2)
+  })
+
+  it('reserves a stable transcript scrollbar gutter for composer width', () => {
+    expect(stableComposerColumns(100, 3)).toBe(93)
+    expect(stableComposerColumns(10, 3)).toBe(20)
+  })
+})
+
 describe('offsetFromPosition — char-wrap inverse of cursorLayout', () => {
   it('returns 0 for empty input', () => {
     expect(offsetFromPosition('', 0, 0, 10)).toBe(0)