OpenCV "namedwindow" goes grey, is not getting events

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I am experimenting with opencv and ROS and Gazebo. This is a variant of an example I found in the Programming Robots with ROS book. I am running on Ubuntu, with Kinetic.

I run the program below, expecting that the window containing the image from the camera on the robot is displayed. It does do this, but after about 10 seconds, the window greys out. I looked at all the logs I can find and see no smoking gun. I have also used rqt_image_view and it does show the image as expected.

Apparently (thanks @gvdhoorn) the greying is because event handling for the graphical window is stalled. To debug this I've added "print n" lines all over the place. The prints show me I think that the loop in the main program "while(not rospy.is_shutdown()):" is not getting a "turn" after a short while, whereas the messages that are coming in via the subscription are getting processed. It is almost as if they are coming so fast that the main program doesn't get a "turn" and so doesn't display the window over and over again. I thought with the "is_shutdown" the processing of the main and the callback would play nice together. Or maybe the problem is something totally different.

Any idea?

import rospy
import cv2
import sys
from sensor_msgs.msg import Image
from cv_bridge import CvBridge, CvBridgeError

# Follower callback
class Follower:
    def __init__(self):
        self.cv_img = None
        # Open CV Bridge, and create a window
        self.bridge = CvBridge()
        # subscribe to the topic, and give each image to the callback
        self.image_sub = rospy.Subscriber('camera/rgb/image_raw', Image, self.image_callback)

    # Called once per image received
    def image_callback(self, frame):
        # Convert image to cv2 and display it
        try:
            print 21
            self.cv_img = self.bridge.imgmsg_to_cv2(frame, desired_encoding="bgr8")
            print 23
        except:
            print("Unexpected error:", sys.exc_info()[0])
            raise

def main():
    cv2.namedWindow('Original')
    print 30
    follower = Follower()
    rospy.init_node('follower', anonymous=True)

    while(not rospy.is_shutdown()):
        print 35
        if follower.cv_img is not None:
            print 37
            cv2.imshow('Original', follower.cv_img)
            print 39
        key = cv2.waitKey(3) & 0xFF
        if key == 27:
            break
        print 43
        rospy.spin()

if __name__ == '__main__':
    main()