/usr/share/fish/completions/htop.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 | # Completions for top
complete -c htop -s d --description "Update interval" -x
complete -c htop -s u --description "Monitor effective UID" -x -a "(__fish_complete_users)"
complete -c htop -l sort-key -d 'Sort column' -xa "(htop --sort-key '')"
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