multiprocessing incorrectly returning an empty array

Question

In this post I asked for some help with multiprocessing as I was struggling to get a return variable and was suggested to check out threading instead.

I found this youtube video looking for something on threading and it that explains that using concurrent.futures I can return a value, this sounds ideal as I need to return a numpy.array.

I've tried simply to run my videoLoop function with concurrent.futures to see what happens before going any further.

This code correctly returns a populated np.array. However, cv.imshow displays once and then closes despite the array returning correctly.

import cv2 as cv
import numpy as np
import concurrent.futures
from PIL import ImageGrab

def videoLoop():
    img = ImageGrab.grab(bbox=(500, 50, 600, 60))
    img_np = np.array(img )
    DetectionWindow= cv.cvtColor(img_np, cv.COLOR_BGR2GRAY)
    cv.imshow("Video Loop", DetectionWindow)
    cv.waitKey(1)
    return DetectionWindow

while True:
    with concurrent.futures.ThreadPoolExecutor() as executor:
        f1 = executor.submit(videoLoop)

I've tried to adapt the above to my actual code and instead I get an array of zeros.

import cv2 as cv
import numpy as np
import concurrent.futures
import pygetwindow
from PIL import ImageGrab


def videoLoop():
    window = pygetwindow.getWindowsWithTitle('Notepad')[0]
    x1 = window.left
    y1 = window.top
    height = window.height
    width = window.width
    x2 = x1 + width
    y2 = y1 + height
    haystack_img = ImageGrab.grab(bbox=(x1, y1, x2, y2))
    (width, height) = (haystack_img.width // 2, haystack_img.height // 2)
    haystack_img_resized = haystack_img.resize((width, height))
    crop = haystack_img_resized.crop((360,22,600,65))
    haystack_img_np = np.array(crop)
    haystack = cv.cvtColor(haystack_img_np, cv.COLOR_BGR2GRAY)
    cv.imshow("Video Loop", haystack)
    cv.waitKey(1)
    return haystack

while True:
    with concurrent.futures.ThreadPoolExecutor() as executor:
        f1 = executor.submit(videoLoop)
        print(f1.result())

What am I missing to return my actual array? and how can I ensure cv.imshow returns a video loop as expected.

Update re: comments

import cv2 as cv
import numpy as np
import concurrent.futures
from PIL import ImageGrab
import pygetwindow

def videoLoop():

    window = pygetwindow.getWindowsWithTitle('Notepad')[0]
    x1 = window.left
    y1 = window.top
    height = window.height
    width = window.width
    x2 = x1 + width
    y2 = y1 + height
    haystack_img = ImageGrab.grab(bbox=(x1, y1, x2, y2))
    haystack_img_np = np.array(haystack_img)
    DetectionWindow= cv.cvtColor(haystack_img_np, cv.COLOR_BGR2GRAY)
    #cv.imshow("Video Loop", DetectionWindow)
    #cv.waitKey(1)
    return DetectionWindow

while True:
    with concurrent.futures.ThreadPoolExecutor() as executor:
        f1 = executor.submit(videoLoop)
        window = f1.result()
        cv.imshow("Video Loop", window)
        cv.waitKey(1)
        print(f1.result())