Cannot center a popper on the screen

CodePen demo

https://codepen.io/anon/pen/WyzXEB

Steps to reproduce the problem

1. Go to the codepen and observe the box is not centered

What is the expected behavior?

I would expect an offset of 50vh and positionFixed: true to allow me to position a popper in the center of the screen, not attached to any specific element, but just attached to html or body.

What went wrong?

The offsets do not work reliably, and there seems to be no way to position a popper always fixed in the center of the screen.

Any other comments?

I really need support for centering a popper in the window, so I can finish converting Shepherd to use popper. Most of it works great, but it also needs to support showing a popper in the middle of the window centered horizontally and vertically, not attached to any elements.

[tminich]

In case someone needs this for Popper v2:

modifiers.push(<ComputeStylesModifier>{
    name: 'computeStyles',
    enabled: true,
    fn({state}) {
        state.styles.popper = {
            ...state.styles.popper,
            position: 'fixed',
            left: `${(window.innerWidth - state.rects.popper.width) / 2}px`,
            top: '50%',
            transform: 'translateY(-50%)',
        }

        return state
    },
})

[rwwagner90]

We ended up using computeStyle for this and manually setting the styles.

computeStyle: {
  enabled: true,
  fn(data) {
    data.styles = Object.assign({}, data.styles, {
      left: '50%',
      top: '50%',
      transform: 'translate(-50%, -50%)'
    });

    return data;
  }
}