Working correctly with box-sizing 'border-box' keeping the original padd...

The way 'box-sizing : border-box' was handled, the original horizontal padding missing inside the pane.

To keep the original padding, the pane is also given the original box-sizing. 
The originalPaddingTotalWidth, which is used for measurements calculations, is set to 0, as it it already accounted for in the paneWidth value.