To manually test this, type some into the input field below and try to undo. The value of the text field should not change after attempting to undo.

initial value:
|   <shadow:root>
|     <div>
|       contenteditable="plaintext-only"
|       "abc"

after prevented undo:
|   <shadow:root>
|     <div>
|       contenteditable="plaintext-only"
|       "abc"