Changing dragmode from 'zoom' to 'pan' makes panning freeze on mousedown on first drag motion

[etpinard]

To replicate:

• Take any cartesian graph e.g. http://codepen.io/etpinard/pen/OMgWjz
• switch layout.dragmode from 'zoom' to 'pan' (via Plotly.restyle or the pan2d mode bar button)
• Try panning the graph. Panning fails on mouse move i.e. the graph doesn't move.
• Release mouse button, the graph pans according to the current mouse location
• Try panning again, panning now works on mouse move.