IME candidate window is misplaced, invisible, or detaches in CJK (Japanese) input

[MtkN1]

Describe the bug

When using Japanese IME input in Copilot CLI, the candidate suggestion popup does not appear immediately below the input box as expected.
Instead, the IME preedit/candidate window appears detached from the main interface (or otherwise fails to track the input line), making Japanese/CJK input difficult to use in Copilot CLI.

This is the same phenomenon as reported in other ink-based CLIs, such as Gemini CLI and Claude Code:

• Gemini CLI: Input with Japanese IME is not working well google-gemini/gemini-cli#1796 (fixed)
• Claude Code: [BUG] IME input causes performance issues and duplicate conversion candidates anthropics/claude-code#1547 (still open)

Screenshots:

• Copilot CLI (problem):

  • While composing Japanese text (e.g., typing “こんにちは”), the IME candidate / conversion UI appears as a detached floating popup near the bottom-left of the terminal window rather than being positioned directly under the Copilot CLI input line.
  • This makes the IME experience confusing and makes it harder to input Japanese/CJK text in the Copilot CLI prompt.

• Gemini CLI (fixed / expected):

  • Under the same environment and IME usage, the IME candidate / conversion UI behaves as expected (i.e., it appears in the correct place relative to the input line and is usable).

Affected version

GitHub Copilot CLI 0.0.418.

Steps to reproduce the behavior

1. Start Copilot CLI in interactive mode
2. Use Japanese (or any CJK) IME in your OS
3. Try to input Japanese text. Observe that the IME candidate popup does not appear below the input box.
4. Input suggestions may instead appear detached (floating or elsewhere), or not at all. Composition buffer/preedit is not displayed in line.

Expected behavior

When inputting CJK text using an IME, the candidate window and composition buffer (preedit) should appear directly under the CLI input field, and input should be usable and perform as expected (same as native terminal apps like bash/zsh/vim).

Additional context

• OS: WSL Ubuntu 24.04.1 LTS
• Copilot CLI: 0.0.418
• Language input: Japanese IME (CJK)
• Terminal: Windows Terminal
• IM framework: Microsoft IME