UE4.16 How to get child widget viewport location? (cached geometry return wrong result)

I have a widget which is a child of a scrollbar.

I want to show a hint UI besides the child widget when my cursor hovered over it.

In order to do so, I need the viewport position of the child widget.

I tried using cached geometry to local viewport as suggested by answer hub, but the return value is wrong if I’m scrolling
and hovered at the same time.

How can I get a scrollbar’s child widget location??