The RadToolTip control has dynamic repositioning mechanism which calculates and sets new position if there is not enough space to be shown without overlapping the target. That is why before scrolling the page it shows above the target and when you scroll the page down, so that there is not enough space for the tooltip, it shows below the target.
However, since you have added OffsetX="-400" , the tooltip is forced to reposition itself again, this time not programmatically because of the repositioning mechanism explained above, but due to the browser boundary detection mechanism. As you see, this scenario with all these settings applied (the negative offset which goes outside the browser in especially) is actually more complicated then it initially seems.
There is also a design problem with this exact scenario - the repositioning is automatic and it is determined by the available space. However, the offset is explicitly set by the developer and at this point we cannot ascertain what the expected behavior should be in such scenario - should the tooltip go out of the screen due to set explicitly, should it take another position, should it take the default position, etc. We are aware that in such a complex case there might be issues with the positioning, but it is very difficult to cover all possible scenarios, especially when the expected behavior differs from one person to another and thus we have provided the client API for fine-tuning.
What I can suggest is the following:
1) Remove the offset or at least make it smaller, since it is actually designed for fine tuning.
2) Set the desired offset only for certain tooltips which really need it in your setup dynamically, through the client API (set_offsetX and set_offsetY methods) .
In order to provide further suggestions, we will need more detailed explanations about the exact desired result and we can try to provide some reasonable solution for you.
the Telerik team