// 自绘滚动条容器：原生滚动条彻底隐藏（零占位——出现/消失永不挤版面，也绕开 Blink
// 对滚动条伪元素动态切换不重绘的 quirk），自绘 thumb 悬浮在内衬/列间隔上。
// 行为契约：鼠标移入滚动区立即现身；移出 / 停止滚动 800ms 后淡出；thumb 可拖拽。
// 用法：外层 .jc-sa 接管原容器的布局角色（className 传 jc-sa--fill / jc-sa--full），
// 原滚动容器的类名整体挪到 vpClassName（视口 = 真正滚动的元素，样式不变）；
// inset = thumb 距右缘 px（负值 = 伸出容器、悬进列间 gutter）。
import { useCallback, useEffect, useRef } from "react";

const HIDE_MS = 800;

export default function ScrollArea({ className = "", vpClassName = "", inset = 3, scrollRef, children }: {
  className?: string;
  vpClassName?: string;
  inset?: number;
  scrollRef?: React.Ref<HTMLElement | null>; // 上层需要直接操作滚动位置时（如会话流自动滚底）
  children: React.ReactNode;
}) {
  const vpRef = useRef<HTMLDivElement | null>(null);
  const thRef = useRef<HTMLDivElement | null>(null);
  const hideT = useRef(0);
  const st = useRef({ inside: false, dragging: false, startY: 0, startTop: 0 });

  // 按滚动几何同步 thumb 高度与位置；不可滚时整体隐藏
  const sync = useCallback(() => {
    const vp = vpRef.current, th = thRef.current;
    if (!vp || !th) return;
    const { scrollTop, scrollHeight, clientHeight } = vp;
    if (scrollHeight <= clientHeight + 1) { th.style.display = "none"; return; }
    th.style.display = "";
    const h = Math.max(28, Math.round((clientHeight / scrollHeight) * clientHeight));
    const y = Math.round((scrollTop / (scrollHeight - clientHeight)) * (clientHeight - h));
    th.style.height = `${h}px`;
    th.style.transform = `translateY(${y}px)`;
  }, []);

  // 移出/停止滚动后延时淡出（悬停内 / 拖拽中不隐藏）
  const hideSoon = useCallback(() => {
    window.clearTimeout(hideT.current);
    hideT.current = window.setTimeout(() => {
      if (!st.current.inside && !st.current.dragging) thRef.current?.classList.remove("on");
    }, HIDE_MS);
  }, []);

  const show = useCallback(() => {
    sync();
    thRef.current?.classList.add("on");
    hideSoon();
  }, [sync, hideSoon]);

  useEffect(() => {
    const vp = vpRef.current;
    if (!vp) return;
    const ro = new ResizeObserver(sync); // 面板拖宽/视口变化时校准
    ro.observe(vp);
    sync();
    return () => { ro.disconnect(); window.clearTimeout(hideT.current); };
  }, [sync]);

  // 标准 ref 转发（写入外部 ref.current 是 ref 的本职），compiler 规则误报
  const setVp = useCallback((el: HTMLDivElement | null) => {
    vpRef.current = el;
    if (typeof scrollRef === "function") scrollRef(el);
    else if (scrollRef) (scrollRef as React.MutableRefObject<HTMLElement | null>).current = el;
  }, [scrollRef]);

  // thumb 拖拽滚动
  const onThumbDown = (e: React.PointerEvent<HTMLDivElement>) => {
    const vp = vpRef.current;
    if (!vp) return;
    e.preventDefault();
    st.current = { ...st.current, dragging: true, startY: e.clientY, startTop: vp.scrollTop };
    thRef.current?.classList.add("on");
    const move = (ev: PointerEvent) => {
      const { scrollHeight, clientHeight } = vp;
      const h = thRef.current?.offsetHeight ?? 28;
      vp.scrollTop = st.current.startTop + (ev.clientY - st.current.startY) * ((scrollHeight - clientHeight) / Math.max(1, clientHeight - h));
    };
    const up = () => {
      st.current.dragging = false;
      window.removeEventListener("pointermove", move);
      window.removeEventListener("pointerup", up);
      hideSoon();
    };
    window.addEventListener("pointermove", move);
    window.addEventListener("pointerup", up);
  };

  return (
    <div
      className={"jc-sa " + className}
      onPointerEnter={() => { st.current.inside = true; show(); }}
      onPointerLeave={() => { st.current.inside = false; hideSoon(); }}
    >
      <div ref={setVp} className={"jc-sa-vp " + vpClassName} onScroll={show}>
        {children}
      </div>
      <div ref={thRef} className="jc-sa-thumb" style={{ right: inset, display: "none" }} onPointerDown={onThumbDown} />
    </div>
  );
}