It used to be the other way around. It’s been a while since I did much UI work, but the web always felt incredibly frustrating for layout compared to Tk. (I think CSS finally has something approaching Tk’s grid layout manager?) There are fewer frameworks for Tk, but I think it has most things you’d need. Maybe some of the finer control over fonts etc is lacking.
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.)
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.
