gh-157210: Attach a combining mark in the last curses Textbox cell

[fedonman]

The lower-right cell of a Textbox is written with insch(), which gave a lone combining mark a cell of its own and pushed the character it belongs to off the window edge. The mark is now merged into the cell that is already there.

No Misc/NEWS.d entry: a combining mark reaches that cell only through the string keystroke path added for 3.16, which has not been released.

• Issue: A combining mark in a Textbox's last cell loses the base character #157210

[serhiy-storchaka]

Thank you for the report and the fix, @fedonman. I continued it in #159651, keeping you as a co-author.

Testing found a few more cases: zero-width marks for which unicodedata.combining() returns 0 (for example Thai U+0E31 or U+20DD) still lost their base, a mark typed into a corner cell that already holds the maximum number of combining characters raised TypeError, and a mark typed after the cursor was moved onto an occupied corner attached to the wrong character. There was also a related bug in insert mode, where a combining character shifted the following text. And since 3.16.0a1 has been released with the string keystroke path, the fix needs a NEWS entry.

Closing this in favour of #159651.