Hello,
This is one of at least 2 bugs I've come across in at least the last 2 or 3 recent versions of PSU.
My current, albeit tedious, workaround is to change the height for the monaco editor from 5px to something more reasonable to be able to edit the code for the App Module. The class list for this element (at least for me) is " monaco-editor no-user-select showUnused showDeprecated"
Let me know if you need more information.