Projection is wrong while dragging and nudging/zooming

[bhousel]

In drag modes, if the user moves towards the edge of the screen, and the nudge happens - or when the user mousewheels - the projection does not update properly. This causes weird jumpiness..

[bhousel]

I did some investigation into this today. I think the issue is that drag.js sets the origin in screen coordinates , which would then go stale if nudges and zooms happen while the drag is active, changing the origin. Instead it should save the origin in world coordinates and project/unproject as necessary.

[bhousel]

fixed in f93e57b Use #map.node() for drag surface instead of svg

The bug was not actually related to the thing I said before with the origin point.
The issue was that #surface can be transformed during a partial redraw
and d3.mouse() was returning transformed point, not the real screen point.
https://github.com/d3/d3-selection/blob/a2cf4f32ec2a88196c9f992e34ec23aec589f797/src/point.js#L4-L9

The draw behavior has similar code, but correctly uses the container object for screen coordinate reference.