Improve ShapeBuilder by adapting the Corel’s SmartFill functionality
Whoever came up with the idea of the ShapeBuilder, they did utilize not that popular LivePaint in a great way. Make it even better!
Steal and add to Ai the Smart Fill from Corel Draw (see the GIF attached). You don’t have to create a live paint, you just click over the intersection, just like ShapeBuilder does, and you have your new shape over the original paths. Steal it and make it better — CorelDraw doesn’t understand click and drag here, and it’s less comfortable to set a color (while Ai won’t have any problem here). Like now ShapeBuilder does nothing when the art is not selected — make it do this instead of nothing.

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Jonah Miller commented
The smart fill tool in Corel is very useful as it creates a separate shape of the fill, so if you have a shape made out of multiple lines, it transforms it into one shape. This makes it easier for laser cutting as it cuts or engraves in one smooth line rather than doing each individual line in the shape separately.
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There's is Live Paint, yes, but it's dynamic, converts original art and can't work without regrouping paths. Interactive tool that just creates new filled objects on intersection could be useful for some users.
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Michael Rescigno commented
we could really use a paint bucket tool, such as the one used in photoshop and MS paint.