Source I guess I was wrong.

File : Source\HelixToolkit.Wpf\Controls\CameraController\RotateHandler.cs

Line 203

        if ((newUpDirection - modUpDir).Length > 1e-8)
        {
            d = 0.2;
        }

a

        if ((newUpDirection - modUpDir).Length < 1e-8)

I guess you're right.

I don't see where you got the first code block from...

The other block looks suspicious, I agree. It is possible this change of up direction is not necessary - but I don't remember what I tried to achieve there. Should have added some more comments...

Do you find the "turnball" :-) mode useful? It was implemented as an experimental mixture of the turntable and trackball rotation modes...