chat-scroll.test.ts

import { describe, expect, it } from "vitest";

import { AUTO_SCROLL_BOTTOM_THRESHOLD_PX, isScrollContainerNearBottom } from "./chat-scroll";

describe("isScrollContainerNearBottom", () => {
  it("returns true when already at bottom", () => {
    expect(
      isScrollContainerNearBottom({
        scrollTop: 600,
        clientHeight: 400,
        scrollHeight: 1_000,
      }),
    ).toBe(true);
  });

  it("returns true when within the auto-scroll threshold", () => {
    expect(
      isScrollContainerNearBottom({
        scrollTop: 540,
        clientHeight: 400,
        scrollHeight: 1_000,
      }),
    ).toBe(true);
  });

  it("returns false when the user is meaningfully above the bottom", () => {
    expect(
      isScrollContainerNearBottom({
        scrollTop: 520,
        clientHeight: 400,
        scrollHeight: 1_000,
      }),
    ).toBe(false);
  });

  it("clamps negative thresholds to zero", () => {
    expect(
      isScrollContainerNearBottom(
        {
          scrollTop: 539,
          clientHeight: 400,
          scrollHeight: 1_000,
        },
        -1,
      ),
    ).toBe(false);
  });

  it("falls back to the default threshold for non-finite values", () => {
    expect(
      isScrollContainerNearBottom(
        {
          scrollTop: 540,
          clientHeight: 400,
          scrollHeight: 1_000,
        },
        Number.NaN,
      ),
    ).toBe(true);
    expect(AUTO_SCROLL_BOTTOM_THRESHOLD_PX).toBe(64);
  });
});