Called during Get2dRectangle, Get2dLine, and GetPoint(..,true) when the mouse down event for
the initial point occurs. This function is not called during ordinary point getting because
the mouse down event terminates an ordinary point get and returns a GetResult.Point result.
RhinoCommon (in RhinoCommon.dll)
Syntax Version Information
Rhino for MacSupported in: 5.4
Rhino for WindowsSupported in: 6.16