Slider curpos might be larger then maxpos while dragging

Steps to Reproduce

https://zkfiddle.org/sample/3lgqeva/1-slider-curpos-dragging-test

open devtool and watch the console
drag the slider button to the right (out of right edge of slider)

Current Result

the _curpos(represent the current value) will exceed the maxpos

Expected Result

not sure this is expected or not

Debug Information

Workaround