Args
ARG(1) indicates which part of the window the mouse is over. It the same value as what you return from your NCHITTEST event handler.
ARG(2) is the X position where the mouse pointer is currently located (referenced from the upper left corner of the desktop screen), and ARG(3) is the Y position.
Notes
This event happens only DBLCLKS option when calling GuiWindowDefaults. The event happens whenever the user presses, releases, and again presses the right mouse button within the system's double-click time limit. Double-clicking the right mouse button actually generates four events: NCRBUTTONDOWN, NCRBUTTONUP, NCRBUTTONDBLCLK, and NCRBUTTONUP again.
If the mouse is captured by some window, then the event happens only with the window capturing the mouse.
See also NCRBUTTONDOWN.