Xournalpp

primarily for note taking, but also usable for filling out PDF forms that don't come with proper input fields

saves inserted text in a separate file

gzipped XML
- copy to .gz → edit in vim → copy to .xopp
could be nicely version controlled using git hooks
- yet to test
- pre-commit hook unzips copy
- post-checkout hook zips

to use with ThinkPad TrackPoint Keyboard II, had to designate it as mouse+keyboard in settings