I am not sure what this "tooltipBorderHack" you are referring to is, but what I can suggest is the following:
1) if possible try to have content with static layout that will fit in the tooltip. The tooltip's size must be a bit larger than the content's because of the paddings, borders and the general UI that is needed.
2) there is a known problem with this property. It is explained here
. A workaround is also available in the feedback page.
All the best,
the Telerik team
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