When entering copy mode in tmux, an indicator in the upper right corner shows "[0/0]". I can't find any documentation in the man page about what these numbers mean, and it's difficult to search since Google throws away the brackets and slash.

This is generated by window_copy_write_line() in window-copy.c:

if (py == 0) {
    size = xsnprintf(hdr, sizeof hdr,
        "[%u/%u]", data->oy, screen_hsize(data->backing));
    if (size > screen_size_x(s))
        size = screen_size_x(s);
    screen_write_cursormove(ctx, screen_size_x(s) - size, 0);
    screen_write_puts(ctx, &gc, "%s", hdr);

but the variable names aren't very instructive to someone who isn't familiar with the code.

Any hints as to what these numbers mean?