REPL import tab completion moves cursor backward

Bug report

Bug description:

When tab completing ambiguous imports in the Python REPL, pressing tab once will insert the longest unambiguous completion and display "[ complete but not unique ]". Pressing tab a second time will show all the possibilities.

In Ghostty, when you press tab the second time, the cursor moves one cell to the left. This happens with cursor-style = bar as well as the default block cursor.

I reproduced this in Ghostty with both fish shell and bash. I could not reproduce this in Zed's integrated terminal (with a bar cursor) or Terminal.app (with a block cursor).

Originally posted by @injust in https://github.com/ghostty-org/ghostty/discussions/11131

I initially reported this issue to Ghostty, and the maintainer's response was:

This is a bug in Terminal.app in how it handles CUB with the cursor in the pending wrap state. Or, a bug in Python's readline in how it calculates the amount to move the cursor back. Your bug reproduces on xterm, and Ghostty matches xterm. Specifically, CUB moves exactly n cells back ignoring pending wrap in xterm and Ghostty matches this. In Terminal.app, CUB subtracts one when it is in the pending wrap state.

Since the bug reproduces in both Ghostty and xterm, it might be a bug in the Python REPL.

I captured the bug in this asciicast: https://github.com/user-attachments/files/25683940/ascii.cast.json

CPython versions tested on:

3.14

Operating systems tested on:

macOS

Linked PRs

• gh-145727

从 Python REPL 中 readline 的 import 补全行为开始，并查看链接的 asciicast，以重现第二次按 Tab 时光标的移动。在终端中比较歧义补全前后的光标位置；当重复进行 Tab 补全不再使光标向后移动时即完成。