Pressing CTRL or Shift+Alt (CMD or Shift on OS X) while clicking and dragging places a crosshair that you can use to move the center of the blur. Because the effect itself is zoomed, the crosshair will move accordingly so you get a nice line that can be tapped to move the center. In addition, you can also shift scroll to get a better view of the entire canvas.
I suppose it's not entirely impossible that these two could be updated to work 50e0806aeb elsderr
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