To manually test this, try to insert text using IME. No text should be committed to the input.

after trying to commit a typed composition:
|   <shadow:root>
|     <div>
|       contenteditable="plaintext-only"
|       <br>

after trying to commit a selected composition:
|   <shadow:root>
|     <div>
|       contenteditable="plaintext-only"
|       <br>