Thanks a lot for your solution. I'm using essential-theme. Only in combination with the following post I found a work-around for this topic.
Cause this is a very important task for all of the teachers, I like to ask if there is a solution where we can use the "splitScreen" even more flexible (allways full width of rubric) ?
Or how to reactivate the buttons at the bottom? Or buttons for 1:4 or 4:1 ratio of the windows?
The Usability is key when Teachers have to review the assignments for the students. And the "Rubric - pdf-marking" is a real USP for Moodle-LMS compared to OneNote or other LMS.
Thank you very much for your help.