Japanese IME composition duplicated when eisuu key ends composition

[kiki-ki]

When using the Japanese IME on macOS, ending composition by pressing the eisuu (英数) key — instead of Enter — causes the composed text to be duplicated.

This is a very common setup among Japanese Mac users: Karabiner-Elements remaps the left Command key (when pressed alone) to send japanese_eisuu, enabling quick IME toggling. This is one of the most popular Karabiner configurations in Japan.

Details

• Browser and browser version: Electron 34.x (VS Code 1.112.0)
• OS version: macOS 15 (Darwin 23.1.0, arm64)
• xterm.js version: @xterm/xterm 6.1.0-beta.191 (bundled with VS Code 1.112.0)

Steps to reproduce

1. On macOS, enable the Japanese IME
2. Install Karabiner-Elements and configure: left Command (alone) → japanese_eisuu
3. Open VS Code's integrated terminal
4. Type Japanese text (e.g. type konnichiha to compose "こんにちは")
5. Press left Command to confirm composition and switch to English input

Expected: "こんにちは" is committed once
Actual: "こんにちはこんにちは" — the text is duplicated

Notes

• Pressing Enter to confirm composition first, then switching input source → no duplication
• Using an external native terminal (e.g. Ghostty) → no duplication
• The issue is not specific to any particular TUI application running inside the terminal

Probable cause

In CompositionHelper.ts, when a non-229 keyCode (such as japanese_eisuu) arrives while _isComposing is true:

1. keydown() calls _finalizeComposition(false) — sends composed text immediately
2. The browser then fires compositionend → calls _finalizeComposition(true) — sends composed text again via setTimeout
3. Both paths emit the same text, resulting in duplication

Native terminal emulators do not have this issue because they use the OS-level IME framework directly, without going through DOM composition events.