How to study Japanese:
1. sign up for a Japanese course at uni
2. obtain a PDF of the textbook
3. find out that you can only see the first page, because under sway 1.0-alpha.6, mupdf doesn't react to any keypresses, so you have no way to switch to the next page
4. find out mupdf still uses X11
5. find out that the bug is related to WM_HINTS
6. learn a bit how WM_HINTS work
7. write a fix, submit a pull request
8. It's merged. It's 1 AM.
It's been fun, but I really need sleep now.