wadd_wch() operates on wrong coords for non-spacing chars

[Roman Zilka]

Hello,

according to curs_add_wch(3x) add_wch() adds non-spacing characters to
the base character on the current coordinates. Therefore, the
following program should print "aáa". It prints "áaa", however.

#include <locale.h>
#include <stddef.h>
#include <stdio.h>
#include <curses.h>

int main() {
    setlocale(LC_ALL, "");
    initscr();

    wchar_t *c = L"a";
    wchar_t *cmod = L"\u0301";
    cchar_t cc;
    cchar_t ccmod;
    setcchar(&cc, c, 0, 0, NULL);
    setcchar(&ccmod, cmod, 0, 0, NULL);

    move(0, 0); add_wch(&cc);
    move(0, 1); add_wch(&cc);
    move(0, 1); add_wch(&ccmod);
    move(0, 2); add_wch(&cc);

    refresh();
    getchar();
    endwin();
}

x86_64-pc-linux-gnu, glibc 2.37, LANG="en_US.UTF-8",
TERM="xterm-256color", ncurses 6.4-20230401 (no relevant NEWS items in
any later version), linking against ncursesw.

No matter what the position is, the non-spacing char is printed one
column too early. It also affects addwstr() et al. If this is indeed
intended, please clarify. I'm not on the mailinglist, but I will watch
the archive.

Discovered while fixing a bug in procps-ng's watch(1) regarding
non-spacing chars:
https://gitlab.com/procps-ng/procps/-/merge_requests/186
commit: "watch: Fixed -d ignoring combining characters [BUG]"

Best regards,
-Roman Žilka