The extrusion function of the shell tool is very powerful and intuitive when modelling geometries but it is limited by the shell focus of the tool.
If it for closed curve profiles was possible to set structure to solid so that instead of extruding a shell with thickness it extruded the closed curve as a solid geometry it would open up for even more uses.
The effect of this can currently be emulated by oversetting the thickness relative to the profile geometry but it tend to produce some geometric artefacts outside the profile and when modifying the profile one also has to make sure that the thickness is correct. Then there's conceptual issue with actually not being a shell with thickness but a solid.
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