[css-overflow] Top layer containing block and scrollbar-gutter #9904
Description
It seems a bit weird to me that a test-case like this live:
<!doctype html>
<html style="scrollbar-gutter: stable both-edges;">
<div style="width: 80ch; outline: auto;">
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
</div>
<dialog id="dialog">
Woo
</dialog>
<button onclick="this.requestFullscreen()">Go fullscreen</div>
<button onclick="dialog.showModal()">Show modal dialog</div>Makes the fullscreen element not cover the whole screen / its containing block origin not start at zero, due to the scrollbar gutters.
Maybe the top layer fixed-pos containing block should actually start at zero / ignore scrollbars on the root?
It's a bit weird for regular fixed-pos elements too, for what is worth, but that behavior is consistent with overflow: scroll at least.
cc: @fantasai @tabatkins @aethanyc @dholbert @smfr @nt1m @mfreed7