We’re doing what? Sounds like you want to drop a widget on the progress bar. Or vice verse. Or is this only about the visual aspect?
Generally speaking, for highlighting during d&d you’d use onDragEnter once a native element is wrapped in a user widget. Since you have a user widget, you can change the visuals of whatever you want inside. For simplicity sake, I’d probably enclose the PB in a padded border and set its brush colour.
if my interface was going to be modular and I wanted maintenance capability, I’d create a user widget with the highlight functionality and expose a Named Slot in which you can now drop any widget.
This way you do not have to laboriously repeat the same script for every widget. And here the answer to the question would be: none. Do not adjust the PB itself.