Composition is cancelled when pressing Enter inside contenteditable

Phenomenon

When using a Korean IME (Input Method Editor) in Firefox on Windows, pressing Enter during composition cancels the composition instead of committing it. This breaks the expected IME workflow.

Reproduction example

1. Create a contenteditable div.
2. Switch to Korean IME.
3. Start typing Korean characters (e.g., “한글”).
4. While the composition is active (characters are still being composed), press Enter.

Observed behavior

• In Firefox on Windows with Korean IME, pressing Enter cancels the composition.
• The partially composed characters are lost.
• The Enter key does not commit the composition as expected.

Expected behavior

• Pressing Enter during IME composition should commit the composed characters.
• The composition should complete and the characters should be inserted into the contenteditable.