>...There were reasons to defer code responsible for
>rebuilding to timer event instead doing it directly
>in OnChangeSelection event. ..
>I disable timer immediately when event is invoked .

I use this technique a lot and it works very well for
me. There is no noticable delay if the interval is
set short enough. However - but You seem to keep an eye
on that already - on very short intervals the timer
tends to fire twice.

I am using a class for that but the principle is
somewhat the same. I use a delay of 20 ms. That
works well and in the first rows of the timer I
do something like

PROCEDURE TIMER
if this.Interval # 0
  this.Interval = 0
else
  return
endif
...

or

if this.Enabled 
  this.Enabled = .F.
else
  return
endif
...

To be sure that the following routine
is called only once.

Maybe this is the reason, why with the wait window
it works better for You than without

HTH