How to show a border around a groupbox when you drag something on it ?

following these discussions :
https://forum.xojo.com/14202-drag-border-vs-focusring-visual-cues-and-drop-file-selected/0
https://forum.xojo.com/27527-drop-highlight-for-a-groupbox

there are examples given to mimic the highlight of a drop element passing on a Canvas.

how can I do it for dropping on a Groupbox ? (which does not have a paint event …)
(other than hiding a canvas of the same size behind the groupbox)

thanks.

The canvas underneath seems to be the way to go.