I was not able to reproduce the problem, but this is the class that is applied to the dropdowns of RadEditor
when the editor is using the Default skin.
My suggestion is to see whether the following style will fix the problem on your end:
z-index: 20000 !important;
If this does not solve the issue, please open a support ticket and send a fully working project. I will examine it and provide a solution.
the Telerik team
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