[BUG] Desktop app (macOS): Japanese IME stops composing while typing fast; keys are inserted as Latin letters though the input source stays Japanese

What's wrong

In the desktop app's chat input (Code tab) on macOS, the Apple Japanese IME intermittently stops composing while I am typing. From that point, keystrokes are inserted as raw Latin letters instead of going into the IME composition. The macOS input menu in the menu bar still shows Japanese (あ), so the input source itself has not changed.

Examples (text left in the input box):

• これは仕様か？watasikoyuunomondaika
• この調査は何度もさせてる。nidotoonajikotosinaideiiyouni

Conditions

• Happens while only typing in the input box, when typing fast. It happens often.
• In both examples the switch happened right after sentence-ending punctuation (？ / 。). Two samples only, so this is not confirmed as a trigger.
• I have only seen it in the Claude desktop app, not in other apps.
• I have no reliable reproduction steps yet.

Expected

Keystrokes keep going through the Japanese IME for as long as the input source is Japanese.

Environment

• Claude desktop app 2.19675.1 (Electron 44.4.3)
• macOS 27.0.1 (26A434), MacBook Pro
• Input sources: ABC and Japanese – Romaji (Apple's built-in IME)
• Secure Input was not active when checked (twice)
• Karabiner-Elements is installed with a rule that sends 英数 / かな when the left / right Command key is pressed alone. That rule changes the input source to ABC, which does not match this symptom (the menu bar stays on あ).

Possibly related

• #83102 (VS Code extension: Korean IME keystroke leaks as raw Latin)
• #93497 (Japanese IME input: mistyped / dropped characters)