.swp files can't be deleted because Vim is always PID 3

If you open `foo.txt` in Vim, it writes `.foo.txt.swp` alongside it. This file is used for crash recovery (among other things) and is removed when you close the file in Vim, or exit Vim cleanly. The file embeds the PID of the Vim process itself.

When you open a file for which a `.swp` file already exists, Vim reads the PID from the file, and checks whether a Vim with that PID is running. If not, it offers you the option to delete the temporary file; if so, it says "(still running)" and won't let you delete it.

However, the Flatpak is always launched in a new PID namespace, so (in practice) it's always PID 3. After a crash, the new Vim process looks for PID 3 and finds itself.