Civi 5.6.0 - wrong css z-index on Activities page when creating/viewing activity
Jern,

This is because of Joomla theme css might be conflicting with CiviCRM css. Try switching the theme to default one if you can replicate the same issue. I tried on my local machine on 5.6.0 and couldn't replicate the issue. Also try to perform CiviCRM cleanup cache if it helps.

Try after reverting the commit in your core file.