Typing Japanese after ASCII characters corrupts prompt layout and text rendering

[snk-k-shibutani]

Describe the bug

When typing Japanese text after ASCII/alphabetic characters in the interactive prompt, the input field layout becomes corrupted and the rendered text no longer matches what was actually typed.

For example, I intended to type:

テストtestテストtest

However, after mixing ASCII text and Japanese text, the prompt line becomes visually misaligned and the rendered text is broken apart with incorrect spacing / cell layout. The issue appears to affect how mixed-width characters are rendered in the prompt input area.

This happens while typing in the interactive UI, before submitting the prompt.

Affected version

GitHub Copilot CLI 1.0.59

Steps to reproduce the behavior

1. Start copilot in the interactive terminal UI.
2. Focus the prompt input field at the bottom of the screen.
3. Type mixed Japanese and ASCII text, for example:
   テストtestテストtest
4. Observe the rendered prompt line while typing.

Expected behavior

The prompt should render the typed text correctly without breaking the layout.

The exact string:

テストtestテストtest

should remain visually intact in the input line, with no extra spacing, broken character cells, or layout corruption.

Additional context

• The issue occurs when Japanese text is entered after ASCII/alphabetic characters.
• This appears to be a mixed-width / IME-related rendering problem in the interactive prompt.
• The attached screenshot shows the corrupted prompt layout after entering mixed Japanese and ASCII text.
• Intended input: テストtestテストtest
• Terminal emulator: Windows Terminal
• Shell: Windows PowerShell
• OS: Windows
• CPU architecture: x86_64