perf(tui): lazily seed virtual history heights (#16523)

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

@@ -0,0 +1,39 @@
+import { describe, expect, it, vi } from 'vitest'
+
+import { ensureVirtualItemHeight } from '../hooks/useVirtualHistory.js'
+
+describe('ensureVirtualItemHeight', () => {
+  it('reuses cached heights without invoking the estimator', () => {
+    const heights = new Map([['a', 7]])
+    const estimateHeight = vi.fn(() => 99)
+
+    expect(ensureVirtualItemHeight(heights, 'a', 0, 4, estimateHeight)).toBe(7)
+    expect(estimateHeight).not.toHaveBeenCalled()
+    expect(heights.get('a')).toBe(7)
+  })
+
+  it('lazily seeds missing heights from the estimator', () => {
+    const heights = new Map<string, number>()
+    const estimateHeight = vi.fn((index: number) => 10 + index)
+
+    expect(ensureVirtualItemHeight(heights, 'b', 2, 4, estimateHeight)).toBe(12)
+    expect(estimateHeight).toHaveBeenCalledTimes(1)
+    expect(estimateHeight).toHaveBeenCalledWith(2, 'b')
+    expect(heights.get('b')).toBe(12)
+  })
+
+  it('falls back to the default estimate when no estimator is provided', () => {
+    const heights = new Map<string, number>()
+
+    expect(ensureVirtualItemHeight(heights, 'c', 0, 4)).toBe(4)
+    expect(heights.get('c')).toBe(4)
+  })
+
+  it('normalizes non-positive estimates to a minimum of one row', () => {
+    const heights = new Map<string, number>()
+    const estimateHeight = vi.fn(() => 0)
+
+    expect(ensureVirtualItemHeight(heights, 'd', 0, 0, estimateHeight)).toBe(1)
+    expect(heights.get('d')).toBe(1)
+  })
+})