This problem is already fixed in 2012 Q1 SP1 version of the controls. As workaround for your current version, you can subscribe to RadRichTextBox.CommandExecuted event and set RadWindow.IsTopmost property to true for the called dialog. For example:
.editor.CommandExecuted += (s, e) =>
RadWindow fontPropertiesDialog = (RadWindow)
I hope this helps. Don't hesitate to contact us if you have other questions.
All the best,
the Telerik team
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