Problem with sit-for and alt-tab in emacs 27

[Richmond]

I have found a problem in sit-for in that it seems to be cancelled by
alt-tab i.e. switch kde windows. This didn't happen in 26.3. I think alt
tab should not get to emacs and I am not sure how it has.

(defun test-sit-for ()
  (interactive)
  (setq quit-for t)
  (while (eq quit-for t)
    (setq nnnn 0)
    (while (and (< nnnn 100) (eq t quit-for))
      (setq quit-for (sit-for 1))
      (message "count %s" nnnn)
      (setq nnnn (+ 1 nnnn))
      )
    )
)

Test with this function, it displays a counter in the message buffer,
but alt-tab away and back and it stops running.