tst.el

(progn
  (defun tst ()
    (macrolet ((with-window-testing-buffer (lines &rest body)
                 (let ((gb (gensym)))
                   `(let ((,gb (get-buffer-create "testing")))
                      (switch-to-buffer ,gb)
                      (erase-buffer)
                      (dotimes (_ ,lines)
                        (insert "a\n"))
                      (unwind-protect
                           (progn ,@body)
                        (kill-buffer ,gb))))))
      (with-window-testing-buffer 500
        (flet ((line-then (line thunk)
                 (progn
                   (goto-line 1)
                   (sit-for 1)
                   (goto-line line)
                   (funcall thunk))))
          (multiple-value-bind (a b)
              (values
               (line-then 500 'window-start)
               (line-then 500
                          (lambda ()
                            (let (r)
                              (run-at-time 0.5 nil
                                           (lambda (w)
                                             (setq
                                              r
                                              (window-start w)))
                                           (selected-window))
                              (sit-for 1)
                              r))))
            (cons (= a b) (cons a b)))))))
  (tst))

;;⇒ (nil 1 . 945)