[oldlaptop]

Most of what I'd call the UI - all the toolbars and pseudo-floating-windows - is basic bread-and-butter Tk stuff. The 3D context and CAD kernel would be the tricky bits. There are extensions floating around to work with OpenGL (or whatever), but I don't see doing the heavy CAD-kernel lifting in Tcl - that would likely have to be in C, or whatever. (Just as it presumably is for Onshape.)

[zvr]

You are correct. Having written such a thing, you do all your basic UI in Tcl/Tk and have a custom widget doing all OpenGL/whatever rendering. The rest of the code treats it as a canvas-like widget.