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

curses.textpad.Textbox.do_command から始め、Linux 上で、提供されている3列および10列の例を使って問題を再現します。記載されている Unicode の動作について Doc/library/curses.rst を確認します。両方のウィンドウが基底文字と結合文字を 'abe\u0301' として保持すれば完了です。