import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";

import { createControlsPanelScrollController } from "./controls-panel-scroll";

const disposers: Array<() => void> = [];
let resize: () => void;
let disconnect: ReturnType<typeof vi.fn>;

function setup(initialScrollTop = 0, height = 1200) {
  const viewport = document.createElement("div");
  viewport.append(document.createElement("div"));
  document.body.append(viewport);
  Object.defineProperties(viewport, {
    clientHeight: { configurable: true, value: 300 },
    scrollHeight: { configurable: true, get: () => height },
  });
  const onScrollTopChange = vi.fn();
  const controller = createControlsPanelScrollController({ viewport, initialScrollTop, onScrollTopChange });
  disposers.push(controller.dispose);
  return {
    viewport, onScrollTopChange,
    grow(next: number) { height = next; resize(); vi.advanceTimersByTime(20); },
    scroll(top: number) { viewport.scrollTop = top; viewport.dispatchEvent(new Event("scroll")); },
    ...controller,
  };
}

beforeEach(() => {
  vi.useFakeTimers();
  disconnect = vi.fn();
  vi.stubGlobal("ResizeObserver", class {
    constructor(callback: () => void) { resize = callback; }
    observe = vi.fn();
    disconnect = disconnect;
  });
});

afterEach(() => {
  disposers.splice(0).forEach((dispose) => dispose());
  document.body.replaceChildren();
  vi.useRealTimers();
  vi.unstubAllGlobals();
});

describe("controls panel scroll controller", () => {
  it("restores immediately without recording a new change", () => {
    const fixture = setup(412.5);
    expect(fixture.viewport.scrollTop).toBe(412.5);
    fixture.scroll(412.5);
    vi.advanceTimersByTime(500);
    expect(fixture.onScrollTopChange).not.toHaveBeenCalled();
  });

  it("retries a clamped offset when content arrives, without persisting interim zero", () => {
    const fixture = setup(650, 300);
    expect(fixture.viewport.scrollTop).toBe(0);
    fixture.scroll(0);
    fixture.grow(550);
    expect(fixture.viewport.scrollTop).toBe(250);
    fixture.scroll(250);
    fixture.flush();
    expect(fixture.onScrollTopChange).not.toHaveBeenCalled();
    fixture.grow(1200);
    expect(fixture.viewport.scrollTop).toBe(650);
    expect(fixture.onScrollTopChange).not.toHaveBeenCalled();
  });

  it("gives user input priority over a pending restore", () => {
    const fixture = setup(650, 550);
    fixture.viewport.dispatchEvent(new Event("wheel"));
    fixture.scroll(100);
    fixture.grow(1200);
    expect(fixture.viewport.scrollTop).toBe(100);
    vi.advanceTimersByTime(150);
    expect(fixture.onScrollTopChange).toHaveBeenLastCalledWith(100);
  });

  it("coalesces rapid scrolling and flushes the latest value once", () => {
    const fixture = setup();
    fixture.scroll(50);
    fixture.scroll(110);
    expect(fixture.onScrollTopChange).not.toHaveBeenCalled();
    fixture.flush();
    expect(fixture.onScrollTopChange).toHaveBeenLastCalledWith(110);
    vi.advanceTimersByTime(500);
    expect(fixture.onScrollTopChange).toHaveBeenCalledTimes(1);
  });

  it("captures a live offset even if pagehide precedes the scroll event", () => {
    const fixture = setup();
    fixture.viewport.scrollTop = 123;
    fixture.flush();
    expect(fixture.onScrollTopChange).toHaveBeenLastCalledWith(123);
  });

  it("flushes the last observed offset on detach, not detached geometry, and cleans up", () => {
    const fixture = setup();
    fixture.scroll(180);
    fixture.viewport.remove();
    fixture.viewport.scrollTop = 0;
    fixture.dispose();
    expect(fixture.onScrollTopChange).toHaveBeenLastCalledWith(180);
    fixture.scroll(250);
    vi.advanceTimersByTime(500);
    expect(fixture.onScrollTopChange).toHaveBeenCalledTimes(1);
    expect(disconnect).toHaveBeenCalled();
  });
});