Indeed, you are correct that this behavior is caused by the fact the RadMenu's z-index is bigger than the one of RadWindow. You can set proper z-indexes in order to get the desired behavior in two manners - to reference the dialog and set z-index or to set lower z-index to the menu. Since all editor's dialogs are RadWindows, the same problem will occur for all of them and thus I believe that the better approach is to set lower z-index to the menu - this is easier solution and it will also solve the situation for any other RadWindows on the page.
You can find a more detailed KB article below:
Information about RadControls' z-indexes values is available below:
I hope that the provided explanations and resources are helpful, let me know how it goes.
All the best,
the Telerik team
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