Changing startMargin causes empty space to appear before the rendered list items

[szszoke]

Describe the bug
Virtualizer doesn't seem to handle when startMargin changes and you get empty space before the rendered list items.

See the video below. It is showing a modified version of the nested example where I am periodically changing the value that is passed to startMargin. This is meant to simulate a page where the value of startMargin depends on a responsive element what might change it's height.

Screencast.from.2024-06-19.17-14-31.webm

To Reproduce
Source code of the modified nested example:

export const Nested: StoryObj = {
  render: () => {
    const ref = useRef<HTMLDivElement>(null);
    const [paddingTop, setPaddingTop] = useState(0);

    useEffect(() => {
      let i = 0;
      const handle = window.setInterval(() => {
        if (i++ % 2 === 0) {
          setPaddingTop(0);
        } else {
          setPaddingTop(400);
        }
      }, 5000);

      return () => {
        window.clearInterval(handle);
      };
    }, []);

    const outerPadding = 40;
    const innerPadding = 60;
    return (
      <div
        ref={ref}
        style={{
          width: "100%",
          height: "100vh",
          overflowY: "auto",
          // opt out browser's scroll anchoring on header/footer because it will conflict to scroll anchoring of virtualizer
          overflowAnchor: "none",
        }}
      >
        <div
          style={{
            backgroundColor: "burlywood",
            padding: outerPadding,
            paddingTop,
          }}
        >
          <div style={{ backgroundColor: "steelblue", padding: innerPadding }}>
            <Virtualizer
              scrollRef={ref}
              startMargin={outerPadding + innerPadding + paddingTop}
            >
              {createRows(1000)}
            </Virtualizer>
          </div>
        </div>
      </div>
    );
  },
};

In reality I am using a resize observer to detect when the size of a div that is rendered before the Virtualizer changes and use its height for startMargin. The initial value is 0 and then it changes to something positive.

Expected behavior
There should not be empty space before the rendered list items.

Platform:

• OS: Linux
• Browser: Firefox and Chromium
• Version of this package: latest main (currently a9d55e2)
• Version of framework: react

Additional context

It is possible to fill in the empty space by increasing the overscan but there is still flashing while the list is scrolled.

[inokawa]

Hi, I'm planning to support dynamic startMargin. It can be tracked in #315