jakoś tak:
GML
var d;
d = point_distance(obj_linia.x,obj_linia.y,mouse_x,mouse_y);
if d<=300
{
draw_line(obj_linia.x,obj_linia.y,mouse_x,mouse_y);
}
else
{
d = point_direction(obj_linia.x,obj_linia.y,mouse_x,mouse_y);
draw_line(obj_linia.x,obj_linia.y,obj_linia.x+lengthdir_x(300,d),obj_linia.y+lengthdir_y(300,d));
}