import turtle

wn = turtle.Screen()
tess = turtle.Turtle()
tess.shape("circle")
tess.color("darkgreen")

# We keep track of "nested event depths" for tess.
# Her drawing on the screen is smooth, but crashes soon.
tess_depth = 0
hightide = 0
def tess_handler(x,y):
    global tess_depth, hightide
    tess_depth += 1
    if tess_depth > hightide:
       hightide = tess_depth
       print(hightide)
    tess.goto(x, y)
    tess_depth -= 1

tess.ondrag(tess_handler)


# We keep track of if the handler is busy for alex, and
# queue events if the previous event has not completed.
# You can see the lag here, but it doesn't crash the system!
alex = turtle.Turtle();
alex.color("purple")
alex.shape("turtle")
alex.forward(20)

alex_handler_busy = False
alex_queued_events = []

def alex_handler(x,y):
    global alex_handler_busy   
    if alex_handler_busy:
        alex_queued_events.append((x, y))
        return
    alex_handler_busy = True
    alex.goto(x, y)
    # Flush extra events out of Alex's queue
    while len(alex_queued_events) > 0:
        (x,y) = alex_queued_events.pop(0)
        alex.goto(x,y)
    alex_handler_busy = False


alex.ondrag(alex_handler)

wn.mainloop()