To make sure the filetype plugin only affects the current buffer use the
:setlocal
command to set options. And only set options which are local to a buffer (see
the help for the option to check that). When using :setlocal for global
options or options local to a window, the value will change for many buffers,
and that is not what a filetype plugin should do.
In ftplugin/zig.vim expandtab, tabstop and shiftwidth are set globally which affects all buffers while it should only be relevant for zig files.
From
:help ftplugin
- http://vimdoc.sourceforge.net/htmldoc/usr_41.html#41.12