Detected error in extreme rare cases for Slider

[David5i6]

I detected some errors in extremes cases for the slider.
Going to pull a merge request with the changes.
Added code guards and documentetation about how to reproduce those cases (It's not easy to reproduce them !).

[David5i6]

Also corrected an error when click on the slider.
If you click outside the handler it move to the end almos always, or to the begining in some cases.
Now it moves to mouse position.