Issue with wbkgrnd and wide chars on ncurses 6.2

[Anton Vidovic]

Hello Thomas,

I contacted you last April [0] about a bug I encountered while working on
a Common Lisp binding for ncurses.

When upgrading from ncurses 6.1 to 6.2 the routine wbkgrnd stopped working.
When called with a wide character, it does not copy the character to every
cell of the terminal.

As far as I could, I tracked down the source of the bug to the following
line 209 in ncurses/base/lib_bkgd.c

for (y = 0; y <= win->_maxy; y++) {
    int x;
    for (x = 0; x <= win->_maxx; x++) {
        NCURSES_CH_T *cp = &(win->_line[y].text[x]);
        int tmp_pair = GetPair(*cp);
        attr_t tmp_attr = AttrOf(*cp);

        if (CharEq(*cp, old_bkgd)) {
>>>>        SetChar2(*cp, CharOf(new_char));    <<<<<<<
        }

In ncurses 6.1, instead of the call to SetChar2, we directly set the
character at y,x to the character set by the call to wbkgrndset.

win->_line[y].text[x] = win->_nc_bkgd;

In addition to the minimal example in Lisp [2] I provided last year,
this would be equivalent minimal example in C:

#define _XOPEN_SOURCE_EXTENDED
#include <locale.h>
#include <ncurses.h>

int main()
{
    cchar_t cch;
    const wchar_t wch[2] = {L'\u2591', L'\0'};

    setlocale(LC_ALL, "");
    setcchar(&cch,
             wch,
             0,
             0,
             NULL);

    initscr();
    bkgrnd(&cch);
    refresh();
    getch();
    endwin();
    printf("%lc\n", wch[0]);
    return 0;
}

This allows setting single-byte chars like 'A' as a background char,
but if any wide char like L'\u2591' is passed, it is simply ignored.

Last year I did not have success replicating the bug in C, but in the
meantime I have upgraded my machine, and now with a fresh install of
Ubuntu 20.04 with a default install of ncurses 6.2.20200212, the C
example replicates the behavior I encountered with Lisp.

I hope you will find an opportunity to address this issue.

Kind regards,
Anton

[0] https://lists.gnu.org/archive/html/bug-ncurses/2020-04/msg00002.html
[1] 
https://github.com/ThomasDickey/ncurses-snapshots/blob/b2401f2eb8036772cc8165a7fd863ad56a868969/ncurses/base/lib_bkgd.c#L209
[2] 
https://github.com/McParen/croatoan/blob/086f34a48f6dbea511ff72b17a9e054635e37e7d/test/unicode.lisp#L121