When combo popups to the top of the page it goes out of the browser viewport.
Reproduce: 
A RadComboBox on the bottom of your page and the page height is larger than the viewport.
When you scroll down the page to the RadComboBox and open it the itembox (100+ items) height will be higher than the view port.
See attachment 'RadComboBox.png'

Also the auto shrink doesn’t work in this situation see attachment RadComboBox2.png