Win8.1x64, 3x gtx580
**Short description of error**
When there is no external variables defined, the driver stops updating in the viewport dynamically. Updates only on more global events like frame change.
**Exact steps for others to reproduce the error**
- Grab/move the object in z axis. It curves proportionally to elevation via driver controlled modifier.
- In the selected driver, remove object reference from variable or the variable itself.
- Grab/move of the object doesn't update modifier effect anymore although the expression is still valid.