test(app): add manual performance diagnostics (#32937)

packages/app/e2e/performance/unit/session-timeline-visual-tracking.test.ts

@@ -0,0 +1,16 @@
+import { expect, test } from "bun:test"
+import { layoutShiftValue, removeVisibleRow } from "../timeline/session-timeline-stream-probe"
+
+test("excludes layout shifts before the probe window and recent input", () => {
+  expect(layoutShiftValue({ startTime: 9, value: 0.1 }, 10)).toBeUndefined()
+  expect(layoutShiftValue({ startTime: 10, value: 0.2, hadRecentInput: true }, 10)).toBeUndefined()
+  expect(layoutShiftValue({ startTime: 11, value: 0.3 }, 10)).toBe(0.3)
+})
+
+test("classifies removed rows from their last painted visibility", () => {
+  const row = {}
+  const visible = new Set([row])
+
+  expect(removeVisibleRow(visible, row)).toBe(true)
+  expect(removeVisibleRow(visible, row)).toBe(false)
+})