[FIX] Message box hiding on mobile view (Safari)

[dougfabris]

Proposed changes (including videos or screenshots)

before

after

Issue(s)

Closes #22252

Steps to test or reproduce

• Open a channel using the safari mobile browser

Further comments

[tassoevan]

This is a recurrent bug. What about checking if the solution from sanitize.css is better?

:where(body) {
  block-size: -moz-available;
  block-size: -webkit-fill-available;
  block-size: stretch;
}