Steps to reproduce the issue

This doesn't happen on 3.8.13
Install Joomla 4.0-dev @ 9d3acfd
Login to Admin -> System -> Site Templates -> Cassiopeia Details and Files -> index.php

Expected result

Be able to see enough of the file to meaningfully edit it.
Only see one editor instance

Actual result

1. only 19 rows of the editor are visible (on my laptop)

2. On resizing the browser from big to small and back to big - I now get THREE editor instances!

System information (as much as possible)

Google chrome 70 on mac

Additional comments