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/lib/precisionWheel.ts

@@ -0,0 +1,48 @@
+const PRECISION_WHEEL_FRAME_MS = 16
+const PRECISION_WHEEL_STICKY_MS = 80
+
+export type PrecisionWheelState = {
+  active: boolean
+  dir: 0 | -1 | 1
+  lastEventAtMs: number
+  lastScrollAtMs: number
+}
+
+export type PrecisionWheelStep = {
+  active: boolean
+  entered: boolean
+  rows: 0 | 1
+}
+
+export function initPrecisionWheel(): PrecisionWheelState {
+  return { active: false, dir: 0, lastEventAtMs: 0, lastScrollAtMs: 0 }
+}
+
+export function computePrecisionWheelStep(
+  state: PrecisionWheelState,
+  dir: -1 | 1,
+  hasModifier: boolean,
+  now: number
+): PrecisionWheelStep {
+  const active = hasModifier || now - state.lastEventAtMs < PRECISION_WHEEL_STICKY_MS
+
+  if (!active) {
+    state.active = false
+
+    return { active: false, entered: false, rows: 0 }
+  }
+
+  const entered = !state.active
+
+  state.active = true
+  state.lastEventAtMs = now
+
+  if (dir === state.dir && now - state.lastScrollAtMs < PRECISION_WHEEL_FRAME_MS) {
+    return { active: true, entered, rows: 0 }
+  }
+
+  state.dir = dir
+  state.lastScrollAtMs = now
+
+  return { active: true, entered, rows: 1 }
+}