In ctags, I have a plan to remove code using 'fpos_t' (https://github.com/universal-ctags/ctags/issues/3727). ctags may use only mio_seek and mio_tell.
AFAICT `fpos_t` does not leak from ctags into Geany, [this](https://github.com/geany/geany/blob/d6ce258cec790da1e81db48476507bb114c7e482...) is the only occurrence in the Geany repository (other than a comment). Perhaps we don't use languages that need it?
So Geany probably doesn't care, but the experts I pinged above should confirm.
Just a comment, `fpos_t` is not simply a [number](https://en.cppreference.com/w/c/io/fpos_t), it is a structure that has other information about the state as well as the offset, so its risky to assign the result of any function returning `fpos_t` to a number or to pass just an offset to any function expecting `fpos_t`. I guess thats why mio defines its position as a union of size_t and `fpos_t`.