I don't think I have the time to look into this. If you'd like to submit a PR with unit tests I'll be happy to review and consider it.

Hey @bvaughn.
I gave a look at this issue, here is what seems to happen:

The property scrollTop was originally a long and was changed into a double. I know chrome implemented the change, but I don't know if firefox did. I can't get a decimal scroll on firefox 67 (according to MDN, decimal happens on systems using display scaling).

So for Firefox, as the value seems to be rounded down, the following check fail:

The rounded value is different from the double one. It triggers a setState and thus call the callback.
This means that an integer scrollTo will work fine, but a decimal scrollTo will trigger the callback multiple time.

On Chrome it is a bit different, if you zoom, you will be able to have decimal scrollTop. However, there seems to be a small offset between the scrollTop and the scroll requested when the check is performed, thus it returns false and triggers a change of the state:

Thanks for sharing your findings, @telaoumatenyanis. Seems like some rounding may be necessary then. PRs welcome for this.

Seems I have somewhat similar issue in firefox, but when using FixedSizeList. Can I help this somehow?

FYI I have a workaround that seems to work in my application. I created an OuterElementType that intercepts the scroll event and rounds all values required by react-windows scroll handler before passing it to said scroll handler. This seems to work well enough for my application, but not sure how testing would go to make sure this works everywhere.

const Outer = React.forwardRef<
  any,
  React.HTMLProps<HTMLElement> & {
    componentType?: ReactElementType;
  }
>(({ onScroll, componentType, ...props }, ref) => {
  const Component = componentType || 'div';

  const handleScroll = (scrollEvent: React.MouseEvent<any>) => {
    // Properties required by inner scroll handler
    // https://github.com/bvaughn/react-window/blob/master/src/createGridComponent.js#L731
    const propertiesToRound = [
      'clientHeight',
      'clientWidth',
      'scrollLeft',
      'scrollTop',
      'scrollHeight',
      'scrollWidth'
    ];
    
    // Passing a fake event with values rounded in order to avoid having multiple scroll events fired if browser is zoomed 
    const fakeEvent: any = {
      currentTarget: {} as any
    };

    for (const property of propertiesToRound) {
      fakeEvent.currentTarget[property] = Math.floor(
        scrollEvent.currentTarget[property]
      );
    }

    onScroll?.(fakeEvent);
  };

  return <Component {...props} onScroll={handleScroll} ref={ref}></Component>;
});