Reverse during animation cause inconsistent state

[simzer]

Reproduce.

1. click on forward (or press right arrow)
2. ckick on backward (or press left arrow) before animation is over.

Result:

• slider jumps, then goes back to the left side
• animation goes forward and finishes on the end stage
• after this point both forward and backward step cause wrong animation

Expected:

• slider does not jump, just goes back
• animation goes back as well