Moveable "jumps" the first time you try and move

[david]

When you use Moveable, in some setups the first time you move a node it "jumps" a little. The problem turns out to be that the body's margin and padding are being taken into account when they really don't need to be. (There's a long comment about how pageX/Y are computed, which may be out of date.)

Two possible fixes are attached.