let w = full_width as isize;
let t = tab_size as isize;
let t_plus_g = t + GUTTER_WIDTH_MIN as isize;      // <-- side_diff.rs:85: overflows
let unaligned_off = (w >> 1) + (t_plus_g >> 1) + (w & t_plus_g & 1);
let off = unaligned_off - unaligned_off % t;       // (also % by t, and t can be 0)

$ printf 'a\n' > f1; printf 'b\n' > f2
$ diff -y --tabsize=9223372036854775805 f1 f2      # overflow-checks build
thread 'main' panicked at src/side_diff.rs:85:24:
attempt to add with overflow
$ echo $?
134