moving rotary point changes other linkage angles

[robz]

Repro:

1. make a fourbar (with points ABCD, where A is the rotary input ground, B is the rotary point, D is the fixed ground point)
2. pause
3. click and drag B to be on the line CD
4. move B across the line CD
5. see that the angle of the triangle ADC changes, instead of being fixed