gh-145448: Fix REPL tab completion cursor position in pending wrap state

When __write_changed_line writes to the last column of the terminal,
the cursor enters "pending wrap" state where it physically stays at
width-1. However, posxy was recording width, causing subsequent CUB
(Cursor Backward) calculations to overshoot by one cell.

Cap posxy x-coordinate at width-1 to match the physical cursor
position in pending wrap state. This fixes the cursor moving backward
during tab completion in xterm and Ghostty.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mvanhorn

Lib/_pyrepl/unix_console.py

@@ -675,7 +675,8 @@ def __write_changed_line(self, y, oldline, newline, px_coord):
             self.__move(x_coord, y)
             self.__write_code(self.ich1)
             self.__write(newline[x_pos])
-            self.posxy = x_coord + character_width, y
+            new_x = x_coord + character_width
+            self.posxy = min(new_x, self.width - 1), y
 
         # if it's a single character change in the middle of the line
         elif (
@@ -686,7 +687,8 @@ def __write_changed_line(self, y, oldline, newline, px_coord):
             character_width = wlen(newline[x_pos])
             self.__move(x_coord, y)
             self.__write(newline[x_pos])
-            self.posxy = x_coord + character_width, y
+            new_x = x_coord + character_width
+            self.posxy = min(new_x, self.width - 1), y
 
         # if this is the last character to fit in the line and we edit in the middle of the line
         elif (
@@ -713,7 +715,14 @@ def __write_changed_line(self, y, oldline, newline, px_coord):
             if wlen(oldline) > wlen(newline):
                 self.__write_code(self._el)
             self.__write(newline[x_pos:])
-            self.posxy = wlen(newline), y
+            # When writing reaches the last column, the terminal enters
+            # "pending wrap" state where the cursor physically stays at
+            # width-1. Record position accordingly so CUB calculations
+            # use the correct physical cursor position (gh-145448).
+            newline_width = wlen(newline)
+            if newline_width >= self.width:
+                newline_width = self.width - 1
+            self.posxy = newline_width, y
 
         if "\x1b" in newline:
             # ANSI escape characters are present, so we can't assume

Misc/NEWS.d/next/Library/2026-03-09-00-00-02.gh-issue-145448.repl-cursor.rst

@@ -0,0 +1,3 @@
+Fixed REPL tab completion cursor moving backward by one cell in terminals
+that correctly handle CUB (Cursor Backward) in pending wrap state, such as
+xterm and Ghostty.