react-virtuoso: overscan props does not work

Question

I'm using react-virtuoso library to render a simple virtual list. The code is very straightforward. I pass this overscan props and expect the virtual list to render n items above and below the viewport but it's not working.

The ExpensiveComponents still renders 'loading...' text when I'm scrolling up and down a little. Here is the code:

import { Virtuoso } from "react-virtuoso";
import { useEffect, useState, useRef, PropsWithChildren } from "react";

function ExpensiveComponent({ children }: PropsWithChildren<{}>) {
  const [render, setRender] = useState(false);
  const mountRef = useRef(false);

  useEffect(() => {
    mountRef.current = true;

    setTimeout(() => {
      if (mountRef.current) {
        setRender(true);
      }
    }, 150);

    return () => void (mountRef.current = false);
  }, []);

  return (
    <div style={{ height: 150, border: "1px solid pink" }}>
      {render ? children : "Loading..."}
    </div>
  );
}

Usage

function App() {
  return (
    <Virtuoso
      style={{ height: "400px" }}
      totalCount={200}
      overscan={3} // ----------> this line does not work
      itemContent={(index) => {
        return <ExpensiveComponent>{index}</ExpensiveComponent>;
      }}
    />
  );
}

Answer 1

I missed this detail from the docs. Unlike react-window API, the overscan unit is pixel instead of row in virtual list, in my case I need to increase the overscan to 900px and it seems to be working now.

<Virtuoso
  style={{ height: "400px" }}
  totalCount={200}
  overscan={900}
  itemContent={(index) => {
    return <ExpensiveComponent>{index}</ExpensiveComponent>;
  }}
/>