I think that adding a pen called highliter is a vital feature for a note-taking application.
The marker works fine on user's handwritten strokes, however, it doesn't work well on PDFs. Screenshot of the issue:

Xournalpp has already this function working fine:

This is one of the main reason I still depend on Xournalpp for editing and annotating PDFs, since highlighting is an important part of my studyflow.