chore: refactoring and tests (#12468)

packages/app/src/pages/session/scroll-spy.test.ts

@@ -0,0 +1,127 @@
+import { describe, expect, test } from "bun:test"
+import { createScrollSpy, pickOffsetId, pickVisibleId } from "./scroll-spy"
+
+const rect = (top: number, height = 80): DOMRect =>
+  ({
+    x: 0,
+    y: top,
+    top,
+    left: 0,
+    right: 800,
+    bottom: top + height,
+    width: 800,
+    height,
+    toJSON: () => ({}),
+  }) as DOMRect
+
+const setRect = (el: Element, top: number, height = 80) => {
+  Object.defineProperty(el, "getBoundingClientRect", {
+    configurable: true,
+    value: () => rect(top, height),
+  })
+}
+
+describe("pickVisibleId", () => {
+  test("prefers higher intersection ratio", () => {
+    const id = pickVisibleId(
+      [
+        { id: "a", ratio: 0.2, top: 100 },
+        { id: "b", ratio: 0.8, top: 300 },
+      ],
+      120,
+    )
+
+    expect(id).toBe("b")
+  })
+
+  test("breaks ratio ties by nearest line", () => {
+    const id = pickVisibleId(
+      [
+        { id: "a", ratio: 0.5, top: 90 },
+        { id: "b", ratio: 0.5, top: 140 },
+      ],
+      130,
+    )
+
+    expect(id).toBe("b")
+  })
+})
+
+describe("pickOffsetId", () => {
+  test("uses binary search cutoff", () => {
+    const id = pickOffsetId(
+      [
+        { id: "a", top: 0 },
+        { id: "b", top: 200 },
+        { id: "c", top: 400 },
+      ],
+      350,
+    )
+
+    expect(id).toBe("b")
+  })
+})
+
+describe("createScrollSpy fallback", () => {
+  test("tracks active id from offsets and dirty refresh", () => {
+    const active: string[] = []
+    const root = document.createElement("div") as HTMLDivElement
+    const one = document.createElement("div")
+    const two = document.createElement("div")
+    const three = document.createElement("div")
+
+    root.append(one, two, three)
+    document.body.append(root)
+
+    Object.defineProperty(root, "scrollTop", { configurable: true, writable: true, value: 250 })
+    setRect(root, 0, 800)
+    setRect(one, -250)
+    setRect(two, -50)
+    setRect(three, 150)
+
+    const queue: FrameRequestCallback[] = []
+    const flush = () => {
+      const run = [...queue]
+      queue.length = 0
+      for (const cb of run) cb(0)
+    }
+
+    const spy = createScrollSpy({
+      onActive: (id) => active.push(id),
+      raf: (cb) => (queue.push(cb), queue.length),
+      caf: () => {},
+      IntersectionObserver: undefined,
+      ResizeObserver: undefined,
+      MutationObserver: undefined,
+    })
+
+    spy.setContainer(root)
+    spy.register(one, "a")
+    spy.register(two, "b")
+    spy.register(three, "c")
+    spy.onScroll()
+    flush()
+
+    expect(spy.getActiveId()).toBe("b")
+    expect(active.at(-1)).toBe("b")
+
+    root.scrollTop = 450
+    setRect(one, -450)
+    setRect(two, -250)
+    setRect(three, -50)
+    spy.onScroll()
+    flush()
+    expect(spy.getActiveId()).toBe("c")
+
+    root.scrollTop = 250
+    setRect(one, -250)
+    setRect(two, 250)
+    setRect(three, 150)
+    spy.markDirty()
+    spy.onScroll()
+    flush()
+    expect(spy.getActiveId()).toBe("a")
+
+    spy.destroy()
+  })
+})