UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36

Steps to reproduce the problem:
1. Go to https://d-toybox.com/studio/lib/input_event_viewer.html
2. Set <textarea> value in "Choose editor type" section to |<p contentediable="false"><span contenteditable>editable span element</span></p>|.
3. Click |<div contenteditable>|
4. Move focus to middle of "editable span element" in the below editor.
5. Press Enter key.

What is the expected behavior?
Inserting <br> *may* be better behavior. And should "undo" restores the original content.

What went wrong?
Chromium creates:
<p contenteditable="false"></p><p contenteditable="false"><span contenteditable="">editable span element</span></p><div></div>
(You can see the result in tooltip when you hover the editor.)

And "undo" does nothing.

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 65.0.3325.181  Channel: stable
OS Version: 10.0
Flash Version: 

Although, this is odd case even in any browsers. Firefox separates the <p> element as well but each <p> has split text at caret position. Edge creates 2 <div> elements under the <span> element. However, in both of them, "undo" works properly.