Undo during IME composition clears more text than expected

Phenomenon

Pressing Undo while an IME composition is active in a contenteditable element removes more text than expected, including characters that were committed before the current composition.

Reproduction example

1. Focus the editable area.
2. Type a short word and finalize it.
3. Activate a Korean IME and start composing another word, but do not finalize it.
4. Press Ctrl+Z (or the platform-specific Undo shortcut).

Observed behavior

• Both the active composition and previously committed characters are removed.
• The event log shows a sequence of beforeinput / input events that do not map cleanly to user intent.

Expected behavior

• Undo reverts only the last committed edit step, or at least behaves in the same way as native controls in the same environment.

Notes

• This behavior can interfere with predictable text editing and undo/redo stacks in products that build their own model on top of contenteditable.