Menu takes a lot of time to open and close on Firefox and Safari

[karlcow]

Steps to reproduce

1. open Firefox Nightly 95
2. reduce the window to something more like a mobile size
3. Go to https://api.cloudflare.com/#getting-started-endpoints
4. Wait for the page to load
5. Click on the top left menu icon (3 bars)
6. wait

Menu is eventually opening
Click again on the top left menu icon
Menu is closing and the page reappears.

This is happening also on Safari mobile, but it is quite slick on Edge Blink

A profile https://share.firefox.dev/3BJ0PEg

This was reported on webcompat/web-bugs#72408
and in https://bugzilla.mozilla.org/show_bug.cgi?id=1735451#c6 where the cause was found.

@emilio found out that

Adding a rule like .site-wrapper { overflow: hidden; } makes it really snappy.

It would be really if it could be fixed.

I tried to find the original CSS in the @cloudflare org but I haven't found yet.
I can send a PR if you point me to the right file.

Thanks.