Drag-drop into text: minor glitches caused by MDL-68113

Since MDL-68113 was done, there are some issues: (there is a useful big-drag-drop-question.xml attached to that issue, wich you can use to test this).

1. If you answer the question using the keyboard (Tab to a drop-zone, and use the arrow keys to cycle through the choices), then if you press left-arrow repeatedly to go thorugh the choices in reverse order, then sometimes the drop boxes disappear.
2. If you press either arrow key very fast, things break (keyboard focus goes away, and the drop box disappears.)
3. Also, when changing items with the keyboard, as well as the old item animating smoothly back to where it came from (which works) the new item should animate smootly into place too (which is not happening). We can live without this last point, but it is nicer if it can work.