The vertical scrollbar displays in the Insert Symbol dropdown menu. It hides the right column of symbols. There are only 20 symbols. The scrollbar is not necessary. I don't know if this is caused by our CSS or some known issue.
We are aware of the InsertSymbol problem. It will be fixed in the next release of the control. For now I can suggest you the following workaround.
In the Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\188.8.131.52__1f131a624888eeed\Resources\ToolImages folder you can find different css files for every Editor's skin e.g RadEditorStyles_Telerik.css. In order to hide the scrollbar you need to add the following lines in the respective
overflow: hidden !important;
All the best,
the Telerik team
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