Backspace removes a whole composed syllable instead of a single character

Phenomenon

When using a Japanese IME in Chrome on macOS, pressing Backspace during composition removes the entire composed syllable (hiragana/katakana) instead of removing individual characters one at a time.

Reproduction example

1. Create a contenteditable div.
2. Switch to Japanese IME.
3. Start typing Japanese characters (e.g., “こんにちは”).
4. While the composition is active, press Backspace.

Observed behavior

• In Chrome on macOS with Japanese IME, Backspace removes the entire composed syllable.
• Individual characters within the syllable cannot be deleted one at a time.
• The granularity of deletion is coarser than expected.

Expected behavior

• Backspace should remove characters one at a time, even during composition.
• Or, the deletion granularity should be consistent and predictable.
• Users should have fine control over text deletion.