If you go to this page:
https://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx
And then go to the HTML tab and pasted this code in:
<div style="border: red 1px solid;" contenteditable="false" unselectable="on">
Non Editable AREA
<div style="border: green 1px solid;" contenteditable="true" unselectable="off">
<!--Content name="info" -->
Editable REGION...
<!--/Content -->
</div>
Non Editable AREA
</div>
Which is from the documentation page:
https://docs.telerik.com/devtools/aspnet-ajax/controls/editor/managing-content/editable-and-non-editable-areas
If you go back to the design tab, you are unable to type anything in the editable area. This works fine in the legacy edge (non-Chromium).
Also interesting is that you can delete things in the non-editable area.