Thank you very much for sharing your thoughts on this wish!
I tested with all construction tools and came to a conclusion: the contour from certain tools can become fill's boundary, while others cannot:
What works as a zone/fill boundary (these do not necessary be in the same plane - only their visible contour in 2D will be enough)
- Curtain wall
What does not work as a zone/fill boundary:
This, I entered a wish (#13028) for better contour detection for Magic Wand.
Thank you once again for sharing the wish! I hope our Product Management team will adopt it soon!
Have a great day!
p/s: certainly 2D elements can also be the fill's boundary in this case, thus, drawing a line can be an (ugly) workaround sometimes.
Technical Support Engineer