fix(tui): steady transcript scrollbar (#20917)

* fix(tui): steady transcript scrollbar

Keep the visible scrollbar tied to committed viewport position while virtual history can still prefetch against pending scroll targets, and preserve drag grab offset synchronously for native-feeling scrollbar drags.

* fix(tui): smooth precision wheel scroll

Replace the opt-scroll throttle with frame-sized coalescing so modifier wheel gestures stay line-precise without stepping.

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

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest'
+
+import { computePrecisionWheelStep, initPrecisionWheel } from '../lib/precisionWheel.js'
+
+describe('precisionWheel', () => {
+  it('passes the first modifier-held wheel event', () => {
+    const s = initPrecisionWheel()
+
+    expect(computePrecisionWheelStep(s, 1, true, 1000)).toEqual({ active: true, entered: true, rows: 1 })
+  })
+
+  it('coalesces same-frame events without throttling line-by-line scroll', () => {
+    const s = initPrecisionWheel()
+
+    computePrecisionWheelStep(s, 1, true, 1000)
+
+    expect(computePrecisionWheelStep(s, 1, true, 1008).rows).toBe(0)
+    expect(computePrecisionWheelStep(s, 1, true, 1016).rows).toBe(1)
+  })
+
+  it('keeps queued momentum in precision mode briefly after modifier release', () => {
+    const s = initPrecisionWheel()
+
+    computePrecisionWheelStep(s, 1, true, 1000)
+
+    expect(computePrecisionWheelStep(s, 1, false, 1050)).toMatchObject({ active: true, rows: 1 })
+  })
+
+  it('leaves precision mode once modifier-free momentum goes idle', () => {
+    const s = initPrecisionWheel()
+
+    computePrecisionWheelStep(s, 1, true, 1000)
+
+    expect(computePrecisionWheelStep(s, 1, false, 1100)).toEqual({ active: false, entered: false, rows: 0 })
+  })
+
+  it('does not coalesce immediate reversals', () => {
+    const s = initPrecisionWheel()
+
+    computePrecisionWheelStep(s, 1, true, 1000)
+
+    expect(computePrecisionWheelStep(s, -1, true, 1008).rows).toBe(1)
+  })
+})