scrollbar-inset-zoom.html

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<!DOCTYPE html>

<html class="reftest-wait">

<!-- The forwarded inset must be a fixed physical size, so full-zooming the

     subdocument must not change it. The reference is the same iframe at zoom 1.

     This only isolates the inset because the subdocument is sized in vh: the

     thumb's device-pixel size and the scrollbar's thickness are already

     zoom-invariant. If the inset were carried in CSS pixels instead of device

     pixels it would double at zoom 2 and the thumb would start lower. -->

<style>

  iframe {

    width: 200px;

    height: 300px;

    border: none;

    -moz-scrollbar-inset-block: 40px 0;

</style>

<iframe src="scrollbar-inset-forward-inner.html"></iframe>

<script>

  const iframe = document.querySelector("iframe");

  iframe.addEventListener("load", () => {

    iframe.frameLoader.browsingContext.fullZoom = 2.0;

    requestAnimationFrame(() =>

      requestAnimationFrame(() =>

        document.documentElement.removeAttribute("class")));

});

</script>