#let cover-with-rect(body, outset: 0.25em, alpha: 70%) = {
  layout(layout-size => {
    style(styles => {
      let body-size = measure(body, styles)
      let bounding-width = calc.min(body-size.width, layout-size.width)
      let wrapped-body-size = measure(box(body, width: bounding-width), styles)
      stack(
        spacing: -wrapped-body-size.height,
        box(body),
        rect(
          fill: rgb(100%, 100%, 100%, alpha),
          width: wrapped-body-size.width,
          height: wrapped-body-size.height,
          outset: outset,
        )
      )
    })
  })
}

#let hider = state("hider", hide)

#let cover-with-style(text-color) = body => {
  text(text-color, body)
}
#let make-text-light-gray = cover-with-style(gray)
#let make-text-dark-gray = cover-with-style(gray.darken(50%))
#let light-mask = cover-with-rect()
#let dark-mask = cover-with-rect(fill: "#0009")

#let update-hider(hide-function) = {
  let wrapper(_) = {
    return hide-function
  }
  hider.update(wrapper)
}

#let _slides-cover(mode, body) = {
    if mode == "invisible" {
        hide(body)
    } else if mode == "transparent" {
        text(gray.lighten(50%), body)
    } else if mode == "hider" {
      locate(loc => {
        hider.at(loc)(body)
      })
    }
    else {
        panic("Illegal cover mode: " + mode)
    }
}

logic.update-hider(logic.make-text-dark-gray)

#logic.polylux-slide[
  Hello #uncover(2)[World]
]