Messages stop sending when it loses focus. Is that normal?

[Tom Glod]

Hi LC Geniuses,

I have a stack that has a handler that sends messages to the stack and
waits until the handler that is triggered completes before it sends the
next one.

It works perfect except for when the stack loses focus. The messages stop
sending.

Is this normal behavior, and i just don't know about it?  Or is this not
really supposed to be happening?

The messages are sent using this format (to keep responsiveness)

  send "DrawCanvasItemTakeSnapshot " & x to stack "UMP Canvas" in 1
milliseconds
  wait until UMPTools["Canvas Viewer"]["Drawing Item"] = 0 with messages


My workaround is to make the window a pallette (solves most of the problem)
or a modal (which solves all of it)....but I don't want a modal window.

Any of you come across this before?  Thanks for any help.