A combining mark in a `Textbox`'s last cell loses the base character

Bug description:

Typing a combining mark into the lower-right cell of a curses.textpad.Textbox puts the mark in a cell of its own and pushes the character it should attach to off the window edge, so that character is lost. The same keystrokes one column earlier attach the mark correctly. Doc/library/curses.rst says of Textbox that "Entering and reading back the full Unicode range, including combining characters, is now supported when curses is built with wide-character support".

import curses, sys
from curses.textpad import Textbox

def typed(ncols):
    win = curses.newwin(1, ncols, 0, 0)
    box = Textbox(win)
    for ch in 'abe\u0301':      # COMBINING ACUTE ACCENT after the 'e'
        box.do_command(ch)
    return win.instr(0, 0, 40).decode()

out = curses.wrapper(lambda stdscr: (typed(3), typed(10)))
print('3 columns:  %a\n10 columns: %a' % out, file=sys.stderr)

Output:

3 columns:  'ab\u0301'
10 columns: 'abe\u0301       '

Expected 'abe\u0301' from both windows: the mark should join the e instead of taking its cell.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-157211

Bắt đầu với curses.textpad.Textbox.do_command và tái hiện vấn đề bằng ví dụ ba cột và mười cột được cung cấp trên Linux. Kiểm tra Doc/library/curses.rst để biết hành vi Unicode đã nêu; hoàn tất khi cả hai cửa sổ đều giữ lại ký tự cơ sở và ký tự kết hợp dưới dạng 'abe\u0301'.