Why is the Locator snapping back to the original coordinates? How can I prevent this?

Question

Take the following code:

p = {3, 3};
Graphics[Circle[{5, 5}, 5], Epilog -> Dynamic@Locator[Dynamic[p]],
 Axes -> True, GridLines -> {{3}, {3}}]

Whenever I move the locator close to the original coordinates (marked by the grid on the plot), it quickly snaps back to them. This means that I am unable to position the locator at around {3.25, 6}. It will either snap to {3.0, 6} or to {3.5, 6}. (Please see the image, it is at {3.5, 6} there.)

Is this a feature or a bug? If it is a feature, how can I turn it off? If it is a bug, how can I work around it?

Note that I am using Epilog because the graphics in my actual problem are slow to compute/render (large ListDensityPlot).

Answer 1

It seems using an EventHandler to simulate a locator will be smooth on my computer ( press Shift instead of press mouse button to active the locator ):

p = {3, 3};
EventHandler[
 EventHandler[
  Graphics[Circle[{5, 5}, 5], Epilog -> Locator[Dynamic[p]], 
   Axes -> True, GridLines -> {{3}, {3}}],
  "MouseMoved" :> Null,
  PassEventsUp :> CurrentValue["ShiftKey"]],
 "MouseMoved" :> (p = MousePosition["Graphics"])
 ]
Dynamic[p]

Update:

I think it's the AlignmentGuidesEnabled feature which causes the snapping.

Here are another two examples which display the problem:

1. Putting the Locator in the "main part" of Graphics instead of in Epilog:

   p = {3, 3};
   Graphics[{Circle[{5, 5}, 5], Locator[Dynamic[p]]}, Axes -> True, 
    GridLines -> {{3}, {3}}]
   

   On my computer, the snapping occurs near the borders of the Graphics but not near {3, 3} anymore.

2. Different Locator actions in one Graphics:

   DynamicModule[{v1 = {2, 0}, v2 = {-1, 1}},
    Dynamic@Graphics[{
       Line[{{0, 0}, v1}],
       Locator[Dynamic[v1]],
       Locator[Dynamic[v2]]
       }, PlotRange -> 3, Frame -> True]]
   

   On my computer (Windows 7 x64, Mathematica 8.0.4), the Locator v1 which linked to the Line moves smooth, but the Locator v2 remains snapping.

So it looks like the behavior of AlignmentGuidesEnabled. Use Ctrl+D open the "Drawing Tools" palette, there is a button at the bottom-left, toggle the AlignmentGuidesEnabled option off, the snapping will gone.