Sorry, if it's already reported somewhere. I can't find it anymore but wonder that it's not fixed already.

Steps to reproduce the issue

• Nightly build of today
• Desktop screen
• Go to Global configuration, e.g. Mail
• Fill the fields with data
• Send a test email

• You'll see a message/alert

• Close it
  

• Select the notify-alert container in browser inspector of Firefox. See blue area on screen:

• Afterwards try to change a field, e.g. in my example above Send Email switcher, that was behind the blue area while inspection. Not possible without reloading the page.

System

Setting Value
PHP Built On Linux dd46134 4.15.0-76-generic #86~16.04.1-Ubuntu SMP Mon Jan 20 11:02:50 UTC 2020 x86_64
Database Type mysql
Database Version 5.7.28-nmm1-log
Database Collation latin1_swedish_ci
Database Connection Collation utf8mb4_general_ci
Database Connection Encryption None
Database Server Supports Connection Encryption Yes
PHP Version 7.4.2
Web Server Apache
WebServer to PHP Interface fpm-fcgi
Joomla! Version Joomla! 4.0.0-beta1-dev Development [ Mañana ] 17-October-2019 20:21 GMT
User Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:73.0) Gecko/20100101 Firefox/73.0