Q:

How to display a popup window on click in PySimpleGUI? 

I am trying to display a popup window that will show the results from an API using PySimple GUI call after the user has selected an item, but nothing is showing when the button is being clicked. 
import PySimpleGUI as sg
# import PySimpleGUIQt as sg
import os.path
import PIL.Image
import io
#import imutils
import requests
import base64
import json
import base64
from detect_face_video  import main
"""

"""

plate='f'

def convert_to_bytes(file_or_bytes, resize=None):
    '''
    Will convert into bytes and optionally resize an image that is a file or a base64 bytes object. 
    Turns into  PNG format in the process so that can be displayed by tkinter
    :param file_or_bytes: either a string filename or a bytes base64 image object
    :type file_or_bytes:  (Union[str, bytes])
    :param resize:  optional new size
    :type resize: (Tuple[int, int] or None)
    :return: (bytes) a byte-string object
    :rtype: (bytes)
    '''
    if isinstance(file_or_bytes, str):
        img = PIL.Image.open(file_or_bytes)
    else:
        try:
            img = PIL.Image.open(io. BytesIO(base64.b64decode(file_or_bytes)))
        except Exception as e:
            dataBytesIO = io. BytesIO(file_or_bytes)
            img = PIL.Image.open(dataBytesIO)

    cur_width, cur_height = img.size
    if resize:
        new_width, new_height = resize
        scale = min(new_height/cur_height, new_width/cur_width)
        img = img.resize((int(cur_width*scale), int(cur_height*scale)), PIL.Image. ANTIALIAS)
    with io. BytesIO() as bio:
        img.save(bio, format="PNG")
        del img
        return bio.getvalue()

def update_plate(filename):

        SECRET_KEY = 'sk_ed3d203bf4c9a7c2910ec0c0'

        with open(filename, 'rb') as image_file:
            img_base64 = base64.b64encode(image_file.read())

        url = 'https://api.openalpr.com/v3/recognize_bytes?recognize_vehicle=1&country=us&secret_key=%s' % (SECRET_KEY)
        r = requests.post(url, data = img_base64)

        try:
             global plate
             plate=r.json()['results'][0]['plate'],
             global Brand
             Brand= r.json()['results'][0]['vehicle']['make_model'][0]['name'],
             global Color
             Color=  r.json()['results'][0]['vehicle']['color'][0]['name'],
             print(plate)

        except:
              print ('error')

# --------------------------------- Define Layout ---------------------------------

# First the window layout...2 columns

sg.theme('Dark Blue 3')
left_col = [[sg. Text('Folder'), sg. In(size=(25,1), enable_events=True ,key='-FOLDER-'), sg. FolderBrowse()],
            [sg. Listbox(values=[], enable_events=True, size=(40,20),key='-FILE LIST-')],
            [sg. Text('Resize to'), sg. In(key='-W-', size=(5,1)), sg. In(key='-H-', size=(5,1))],
            [sg. Button("Resize", button_color=("white", "blue"), size=(6, 1))]]

# For now will only show the name of the file that was chosen
images_col = [[sg. Text('You choose from the list:')],
              [sg. Text(size=(40,1), key='-TOUT-')],
              [sg. Image(key='-IMAGE-')]]

# ----- Full layout -----
layout = [[sg. Column(left_col, element_justification='c'),  sg. VSeperator(),sg. Column(images_col, element_justification='c')]]

# --------------------------------- Create Window ---------------------------------
window = sg. Window('Multiple Format Image Viewer', layout,resizable=True)

# ----- Run the Event Loop -----
# --------------------------------- Event Loop ---------------------------------
while True:
    event, values = window.read()

    if event in (sg. WIN_CLOSED, 'Exit'):
        break
    if event == sg. WIN_CLOSED or event == 'Exit':
        break
    if event == '-FOLDER-':                         # Folder name was filled in, make a list of files in the folder
        folder = values['-FOLDER-']
        try:
            file_list = os.listdir(folder)         # get list of files in folder
        except:
            file_list = []
        fnames = [f for f in file_list if os.path.isfile(
            os.path.join(folder, f)) and f.lower().endswith((".png", ".jpg", "jpeg", ".tiff", ".bmp"))]
        window['-FILE LIST-'].update(fnames)
    if event == 'Resize':
        window['-IMAGE-'].update(data=convert_to_bytes(filename, resize=new_size))
    elif event == '-FILE LIST-':    # A file was chosen from the listbox
        try:
            filename = os.path.join(values['-FOLDER-'], values['-FILE LIST-'][0])

            window['-TOUT-'].update(filename)

            if values['-W-'] and values['-H-']:
                new_size = int(values['-W-']), int(values['-H-'])

            else:
                new_size = None

            window['-IMAGE-'].update(data=convert_to_bytes(filename, resize=new_size))

            main(filename)
            update_plate(filename)
            

            print(plate)
        except Exception as E:
            print(f'** Error {E} **')
            pass        # something weird happened making the full filename
# --------------------------------- Close & Exit ---------------------------------
window.close()

A:

Setting keep on top in the Popup call created the window on top for me.  Just add
sg. Popup("license plate" , plate , keep_on_top=True)

However, if you click on the window behind, because it also has keep on top set, it will cover your popup.