Vector Fields, Divergence and Curl, and Flux and Circulation Integrals (robphy)
The latest 2023 version now does a calculation with the path for divergence as flux/vol and curl as circulation/area (the other moveable div and curl meters used the symbolic formulas).
You can modify the various paths. In some cases, you may be able to rigidly move the path by grabbing the faint grey line joining two control points.
The original 2019-version is based on https://www.geogebra.org/m/XfmAAUTG
(Vector Fields, Divergence, Curl, and Line Integrals by Erik Jacobsen)
which is claimed to be
based on (presumably) https://www.geogebra.org/m/UstetaTp
(Vector Fields and Line Integrals by George Sturr).
The "Purcell" vector fields are based on E.M. Purcell's Electricity and Magnetism textbook.