It all started with a sketch posted by @dimitar here: https://forum.freecadweb.org/viewtopic.php?f=3&t=39276

I thought I could fix the problem by scaling constraint error functions, to make them all return values of similar magnitudes.

So, I made this:

https://github.com/DeepSOIC/FreeCAD-ell ... ntScaling1

I scaled down all error functions by dividing stuff by the size of the sketch.

Unfortunately, the results are underwhelming. While the sketch by @dimitar is successfully solved with the changed constraint definitions, there is a big new problem: dragging arcs of circles is now very very jittery on any sketch of substantial size.
So, my guesses:

* (1) the solver uses some junk from memory

or

* (2) the solver is for some reason very unstable with this scaling applied

I have added some debug output for scaling calls, these seem to be all right. So I think (1) isn't the problem.

As for (2): maybe, the solver changes angle-like unknows way too fast. I just have no idea really. Maybe I should be scaling unknowns so that they are all in the same ballpark instead of scaling the errors. So, a thing to try is to simply scale the whole sketch down so that it is around 1 mm in size for solving purposes.