If misspelled word is coming after scroll bar it is not focusing in IE11, Edge and Chrome.

Can be reproduced in the default demo - https://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx

Paste the following content 

<p>a</p>
<p>a</p>
<p>a</p>
<p>a</p>
<p>a</p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p></p>
<p>risksss</p>

and click on the spell check button. The popup with the suggestions will be positioned outside of the content area.

As a workaround you can switch to DIV content area mode - https://demos.telerik.com/aspnet-ajax/editor/examples/contentareamodediv/defaultcs.aspx