This function will return the mouse y position within the
screen. It should be noted that this function only works
properly when used on the Windows target. It can be used for HTML5
too, but will only return a value relative to the 0,0 of the
canvas itself, and will not return any value while the mouse is
outside of the canvas. For other devices it will return 0, and you
should use the
device_mouse_raw_y functions instead.
my_y = display_mouse_get_y();
This would set my_y to the y coordinate of the mouse relative to the display.