You are overriding implemented functions in Ext.panel.DD
You should only need draggable: true
Having said that, there's a slight bug with synching the panel position with the position of the dragged proxy upon mouseup if the Panel is it's default hideMode, 'display'.
I submitted a fix, but unfortunately, it's too late to get into beta 1. It will be fixed in 4.1.0 final.
A workaround is to configure the Panel hideMode: 'offsets'