Currently, when I set background-iframe, the site is not scrollable. 

Reprex:

```
---
title: "Untitled"
format: revealjs
---

##  {background-iframe="https://quarto.org"}
```

My currently workaround is 
```
##  {fullscreen="true"}

<iframe class="stretch" data-src="https://quarto.org/" title="https://quarto.org/">

</iframe>
```
which is basically what I want. Not sure how trivial or difficult it would be to fix, but I thought I'd note it here.

2022-07-30T20:14:17Z
trangdata
enhancement
Future
open
Background iframe in revealjs is not scrollable
https://github.com/quarto-dev/quarto-cli/issues/1603