Trying to move and losing it

I occasionally find that if I am offsetting a single straight line then I can’t get it to copy properly:

(note this only happens about one in twenty times for me and the following is the only common work-flow I have found it happens on…)

  • Draw a cube, draw a box on a face, push it in by a bit. Select the bottom line inside the inset [click].
  • Move the mouse upwards so that you can see the line moving up, parallel to the lower edge (can even use the arrow keys).
  • Type in a value and hit [return]
  • nothing happens.

as a work-around I have to [click] to place the line, then type in the value to get the precision I want.