If you want to ensure this is not visible when you show the control, you could change the template of RadRichTextBox. Alternatively, you could force a measure of the control once it is loaded and everything should work fine.
sorry for posting in this old thread, but I have the same problem with that control. But it's even worse.
My control ist very narrow (only one line is visible).
When this gray square is shown, the text of the RichTextBox is also moved up (shown with some margin). This leads to the effect, that my text is outside of the users view (see the attached picture. On the screenshot you can only see the red underline of the spell checker. The text itself is hidden, it's above).
I tried to invoke a measure of the control, like you said which really made the gray square disappear. The text however remains moved up.
As soon as I start typing (changing the preset text) the text comes down and I can see it (See second picture).
How can I workaround this strange behavior?
If I need to override the ControlTemplate, could you please provide it, so that I can directly copy the xaml?
Thank you very much.