Checking if mouse pointer is exactly over table row (without offset)

[piotr-przebieracz]

Currently, when we drag a row by clicking on the top of the row, the drag class is applied to the next row before mouse pointer is really over this row. Furthermore, when we drag a row by clicking on the bottom of the row, the drag class is applied to the next row some time after mouse pointer crossed border of this row. Is this behaviour intentional? I thought that in some cases better solution could be to apply the drag class right after the moment when mouse pointer crossed border of the row (not taking into account if the user started dragging at the top or at the bottom of the row). In this pull request i proposed a change which enable to choose between those two behaviours: calculating mouse offset of the initial mouse pointer position (current behaviour) and the second, which I described above.