perf(tui): stabilize long-session scrolling

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

@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest'
+
+import { getViewportSnapshot, viewportSnapshotKey } from '../lib/viewportStore.js'
+
+describe('viewportStore', () => {
+  it('normalizes absent scroll handles', () => {
+    expect(getViewportSnapshot(null)).toEqual({
+      atBottom: true,
+      bottom: 0,
+      pending: 0,
+      scrollHeight: 0,
+      top: 0,
+      viewportHeight: 0
+    })
+  })
+
+  it('includes pending scroll delta in snapshot math and keying', () => {
+    const handle = {
+      getPendingDelta: () => 3,
+      getScrollHeight: () => 40,
+      getScrollTop: () => 10,
+      getViewportHeight: () => 5,
+      isSticky: () => false
+    }
+
+    const snap = getViewportSnapshot(handle as any)
+
+    expect(snap).toMatchObject({ atBottom: false, bottom: 18, pending: 3, scrollHeight: 40, top: 13, viewportHeight: 5 })
+    expect(viewportSnapshotKey(snap)).toBe('0:13:5:40:3')
+  })
+})