Just passing along a bug that I found with regards to tooltip positioning and IE 11 (I didn't check other versions of IE and it does not happen in Chrome or Firefox).
If your <body> element is scrollable (vis overflow: auto and a fixed width/height), once you scroll, the tooltip position will be off. The reason is buried deep with Telerik's script code.In PopupScripts.js, the _fixPositionInBounds function, called when the popup tooltip is being shown, the code calls _getViewportBounds, which is this:
For IE, scrollTop for <body> will be non-0 when scrolled which causes Telerik's popup script to think the tooltip will be positioned off screen so it adjusts. Unfortunately, this adjustment puts the tooltip in the wrong place (vertically much lower than it should be).
The simple fix for me was to put a containing <div> as a child of <body> and use that <div> to scroll. After I did that all is well. It took me 2 days to figure this out, so posting this here :)