This is a browser behavior. The HTML based dialogs of RadEditor are positioned below the Windowed flash and media objects, because the windowed objects have larger z-index.
If you don't like this behavior you can configure the editor to use the browser modal
dialogs instead of RadWindow using the following code:
|<telerik:RadEditor ID="RadEditor1" OnClientLoad="OnClientLoad" runat="server"></telerik:RadEditor>
You can find more information in this KB article: Using browser modal dialog instead of RadWindow
All the best,
the Telerik team
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