This may happen when the tooltip does not have a proper size set. In its show event it positions itself according to the initial size it has, then it has to resize when the whole response is received. At this point, though, the callout and position are already set and thus they appear wrong when the tooltip repositions itself so that it stays in the screen.
active = Telerik.Web.UI.RadToolTip.getCurrent();
width = active._tableElement.offsetWidth;
height = active._tableElement.offsetHeight;
Where these functions are attached to the OnClientResponseEnd and OnClientShow events respectively.
Please note that this hack may not function well with animations.
the Telerik team
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