Thank you for reporting this problem. As a small act of gratitude, I will update your telerik points.
Since the Service Pack for Q2 2011 is already knocking on the door, I cannot commit on releasing the fix for this problem by this time. But as soon as we are ready, you will be able to get the fix in a Latest Internal Build.
You will be able to fix it, by calling the repaint method of the RadEditor's client object on change of tab selection client event. This is required, since RadRibbonBar needs to know sizes when initialized on the client, and when the content is out of the page's flow (the container is set with style display: none) RibbonBar cannot properly initialize.
For now you can use the RenderSelectedPageOnly="true" on RadMultiPage in order to always render the RibbonBar visible and therefore resized properly.
We will notify you, as soon as we are ready with the fix.
the Telerik team
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