React Material-UI menu anchor broken by react-window list

I am using Material-UI and react-window in a project. My issue is, the material-ui menu component does not anchor to the element provided when that element is within a react-window virtualized list. The menu will appear in the upper left corner of the screen instead of anchored to the button that opens it. When using it all in a non-virtualized list, it works as expected. The menu properly anchors to the button that opens it.

Here's an example sandbox. The sandbox is pretty specific to how I'm using the components in question.

Any guidance on how I can resolve this?

Here's a modified version of your sandbox that fixes this:

Here was your initial code in BigList:

const BigList = props => {
  const { height, ...others } = props;
  const importantData = Array(101 - 1)
    .fill()
    .map((_, idx) => 0 + idx);
  const rows = ({ index, style }) => (
    <FancyListItem
      index={index}
      styles={style}
      text="window'd (virtual): "
      {...others}
    />
  );

  return (
    <FixedSizeList
      height={height}
      itemCount={importantData.length}
      itemSize={46}
      outerElementType={List}
    >
      {rows}
    </FixedSizeList>
  );
};

I changed this to the following:

const Row = ({ data, index, style }) => {
  return (
    <FancyListItem
      index={index}
      styles={style}
      text="window'd (virtual): "
      {...data}
    />
  );
};

const BigList = props => {
  const { height, ...others } = props;
  const importantData = Array(101 - 1)
    .fill()
    .map((_, idx) => 0 + idx);
  return (
    <FixedSizeList
      height={height}
      itemCount={importantData.length}
      itemSize={46}
      outerElementType={List}
      itemData={others}
    >
      {Row}
    </FixedSizeList>
  );
};

The important difference is that Row is now a consistent component type rather than being redefined with every render of BigList. With your initial code, every render of BigList caused all of the FancyListItem elements to be remounted rather than just rerendered because the function around it representing the "row" type was a new function with each rendering of BigList. One effect of this is that the anchor element you were passing to Menu was no longer mounted by the time Menu tried to determine its position and anchorEl.getBoundingClientRect() was providing an x,y position of 0,0.

You'll notice in the react-window documentation (https://react-window.now.sh/#/examples/list/fixed-size) the Row component is defined outside of the Example component similar to how the fixed version of your code is now structured.