CODE
draw_line(mouse_x,mouse_y,room_width,mouse_y)
draw_line(mouse_x,mouse_y,mouse_x,room_height)
draw_line(mouse_x,mouse_y,mouse_x,0)
draw_line(mouse_x,mouse_y,0,mouse_y)
draw_line(mouse_x,mouse_y,mouse_x,room_height)
draw_line(mouse_x,mouse_y,mouse_x,0)
draw_line(mouse_x,mouse_y,0,mouse_y)
This draws 4 lines, connecting the top,bottom, and both sides to the mouse.