When you set the ContentUrl of the RadPane to an external page, the RadPane creates an IFRAME element and opens the page in that IFRAME. That is why, this case, you can refer to the RadPane as an IFRAME.
I would suggest that you test the page with the Paypal button in a simple IFRAME element to see if you can reproduce the problem with it. In case you cannot, please prepare and send me a simple running project, demonstrating the problem and I will research what is causing it. However, in case you can reproduce the problem with a simple IFRAME element, I suppose the problem is in your implementation or Paypal rather than in the code of the RadSplitter control.
All the best,
the Telerik team
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