Thanks for using DropBox. Very much appreciated.
I would use the Field Option in the Selection Tag to get this result.
If you use a gradient, you need to use the Fields> Tolerance> % to create a selection. The default is 50%.
You can effortlessly animate this, based on Fields. Press play to see the result.
Let me know if there is anything else. I’m happy to look into it.
All the best
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